were protected after a single dose of the Pfizer vaccine and only 27 percent after a single dose of the AstraZeneca vaccine. (In Britain, the current practice is to delay second doses to stretch vaccine availability.)

Likewise, another study published last month indicated that fewer than 15 percent of patients with cancers of blood or the immune system, and fewer than 40 percent of those with solid tumors, produced antibodies after receiving a single dose of the Pfizer-BioNTech vaccine.

And a study published last month in the journal JAMA reported that only 17 percent of 436 transplant recipients who got one dose of the Pfizer-BioNTech or Moderna vaccine had detectable antibodies three weeks later.

Despite the low odds, immunocompromised people should still get the vaccines because they may produce some immune cells that are protective, even antibodies in a subset of patients.

“These patients should probably be prioritized for optimally timed two doses,” said Dr. Tariq Ahmad, a gastroenterologist at the Royal Devon and Exeter NHS Foundation Trust who was involved in the infliximab studies.

He suggested that clinicians routinely measure antibody responses in immunocompromised people even after two vaccine doses, so as to identify those who also may need monoclonal antibodies to prevent infection or a third dose of the vaccines.

Wendy Halperin, 54, was diagnosed at age 28 with a condition called common variable immunodeficiency. She was hospitalized with Covid-19 in January and remained there for 15 days. But the coronavirus induced unusual symptoms.

“I was having trouble walking,” she recalled. “I just lost control of my limbs, like I couldn’t walk down the street.”

Because she was treated for Covid-19 with convalescent plasma, Ms. Halperin has had to wait three months to be immunized and has made an appointment for April 26. But despite her condition, her body did manage to produce some antibodies to the initial infection.

“The take home message is that everybody should try and get the vaccine,” said Dr. Amit Verma, an oncologist at Montefiore Medical Center.

The gamble did not pay off in Dr. Wollowitz’s case. Without antibodies in his system to protect him, he is still working from home — a privilege he is grateful for. He was an avid mountain biker and advanced skier, both of which carry risk of injury, but with the coronavirus, he is playing it safe.

In anticipation of returning to his normal lifestyle, Dr. Wollowitz is tuning his bicycles. But he said he foresaw himself living this way till enough other people are vaccinated and the number of infections in the city drops.

“I’m not exactly sure what that date is,” he said. “I’m really waiting to get back out.”

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Top health officials urge Americans to get vaccinated but barely address the J.&J. dose pause.

Three top federal health officials appeared on Capitol Hill on Thursday and implored Americans to get vaccinated against the coronavirus, but said little about the investigation into whether the Johnson & Johnson vaccine may be linked to a small number of cases of rare blood clots, or when that vaccine might be put back into use.

“Hopefully we’ll get a decision quite soon, as to whether or not we can get back on track with this very effective vaccine,” Dr. Anthony S. Fauci, President Biden’s top medical adviser for the coronavirus, told a House panel.

Dr. Fauci’s comments came as the future of the Johnson & Johnson one-shot vaccine hung in the balance. Earlier this week, the Food and Drug Administration and the Centers for Disease Control and Prevention called for a pause in the use of the vaccine in the wake of reports of a small number of rare blood clots in recipients. Though it is unclear whether the vaccine was responsible for the clots, injections came to a sudden halt across the country.

On Wednesday, a C.D.C. advisory panel suggested that it would be a week to 10 days before they had enough information to assess the vaccine’s risks and could make a decision about its future. In the meantime, the officials — Dr. Fauci; Dr. Rochelle Walensky, the C.D.C. director; and Dr. David Kessler, who runs the Biden administration’s vaccine effort — urged Americans to continue to get vaccinated.

an immediate pause in the use of Johnson & Johnson’s single-dose Covid-19 vaccine after six recipients in the United States developed a rare disorder involving blood clots within one to three weeks of vaccination.

  • All 50 states, Washington, D.C. and Puerto Rico temporarily halted or recommended providers pause the use of the vaccine. The U.S. military, federally run vaccination sites and a host of private companies, including CVS, Walgreens, Rite Aid, Walmart and Publix, also paused the injections.
  • Fewer than one in a million Johnson & Johnson vaccinations are now under investigation. If there is indeed a risk of blood clots from the vaccine — which has yet to be determined — that risk is extremely low. The risk of getting Covid-19 in the United States is far higher.
  • The pause could complicate the nation’s vaccination efforts at a time when many states are confronting a surge in new cases and seeking to address vaccine hesitancy.
  • Johnson & Johnson has also decided to delay the rollout of its vaccine in Europe amid concerns over rare blood clots, dealing another blow to Europe’s inoculation push. South Africa, devastated by a more contagious virus variant that emerged there, suspended use of the vaccine as well. Australia announced it would not purchase any doses.
  • The reports of blood clots were the second recent blow to the Johnson & Johnson vaccine. Earlier this month, an ingredient mix-up at a Baltimore manufacturing plant owned by Emergent BioSolutions ruined up to 15 million doses of the vaccine. The F.D.A. is now inspecting the plant to see whether any vaccine doses manufactured there can be released to the public.

    About 7.7 million Americans had received the Johnson & Johnson vaccine as of Wednesday, accounting for less than 4 percent of the more than 198 million doses administered across the country. The Pfizer and Moderna vaccines are in much greater supply.

    Officials note that the blood clots are extremely rare; the handful of cases represent less than one in one million recipients, although that incidence estimate could go up if more cases are reported.

    Biden administration officials say that the absence of the Johnson & Johnson vaccine might not have a major impact on the U.S. vaccination campaign. But if use of the vaccine is severely restricted worldwide, it could prove disastrous for the global vaccination effort.

    Health officials had hoped that the Johnson & Johnson vaccine, along with a similar vaccine developed by AstraZeneca, would help supply the world because they are less expensive and easier to store and handle than the Pfizer and Moderna vaccines.

    Denmark, where two recipients suffered severe blood clots, permanently suspended use of the AstraZeneca vaccine.

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    C.D.C. Panel Keeps Pause on Use of J&J Vaccine, Weighing Risks

    “Right now, we believe these events to be extremely rare, but we are also not yet certain we have heard about all possible cases, as this syndrome may not be easily recognized as one associated with the vaccine,” Dr. Rochelle P. Walensky, the C.D.C. director, said at a White House news conference on the pandemic on Wednesday.

    During the panel discussion, experts noted that the “risk window” for the condition among vaccine recipients was still open and that new cases might emerge, because nearly 3.8 million people had received the shot within the last two weeks. In the six women, the severe clotting developed within about two weeks of the shot.

    Other experts encouraged dissemination of health information on diagnosis and treatment of the condition so that awareness would spread among doctors, emergency rooms and people who had received the vaccine. A key point is that the blood-thinner heparin, a common treatment for clots, can harm these patients and should not be used.

    Officials also noted that people with the condition needed to be treated as soon as possible because the clots were so serious. Some patients needed invasive procedures to remove large clots from blood vessels in their brains.

    Several panel members reiterated that two other vaccines — from Moderna and Pfizer-BioNTech — are available, neither associated with the clotting problem, so continuing the pause would not stop most people in the United States from being vaccinated.

    At the news conference, Jeffrey D. Zients, the White House’s pandemic coordinator, said that the pause would not generally interrupt the momentum of the country’s vaccination campaign.

    “In the very short term, we do expect some impact on daily averages as sites and appointments transition from Johnson & Johnson to Moderna and Pfizer vaccines,” he said. “We have more than enough Pfizer and Moderna vaccine supply to continue or even accelerate the current pace of vaccinations.”

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    Expert Panel to C.D.C. to Vote on Johnson & Johnson Vaccine Pause

    An advisory committee for the Centers for Disease Control and Prevention is discussing the Johnson & Johnson Covid-19 vaccine pause during a meeting on Wednesday afternoon while a possible link to a small number of rare blood clots is investigated.

    The emergency meeting follows the Food and Drug Administration’s announcement on Tuesday that it was studying six cases of rare and severe blood clots in women aged 18 to 48, one of whom died. All of the women had received the Johnson & Johnson vaccine before developing the clots, though it is unclear whether the vaccine is responsible. As of Tuesday, more than seven million people in the United States have received the shot, and another 10 million doses have been shipped out to the states, according to C.D.C. data.

    Following the call from federal health agencies, all 50 states, Washington, D.C., and Puerto Rico on Tuesday quickly paused or recommended that providers pause the administration of the vaccine. The U.S. military, federally run vaccination sites, and a host of private companies, including CVS, Walgreens, Rite Aid, Walmart and Publix also paused the injections.

    The Advisory Committee on Immunization Practices, or ACIP, is a panel of independent experts who advise the C.D.C. on its vaccine policies. At the meeting, the experts are reviewing and debating data from the rare cases, and will later hear comments from the public, before a possible vote on how to proceed. They could vote to recommend that the pause continues, for example, or to specify that it should apply only to a certain age or sex.

    reiterated on Wednesday that the pause in Johnson & Johnson vaccinations gives public health officials a chance to investigate the cases and discuss them with health care professionals. He added that pauses are common when new vaccines and drugs are rolled out.

    “We’re just doing the due diligence we need to do to make sure everything is safe so we can continue with our vaccination efforts,” Dr. Murthy said on “CBS This Morning.”

    was not worth the trade-off of slowing the vaccination campaign and potentially eroding the public’s trust of vaccines in general.

    At the news conference, Jeffrey D. Zients, the White House’s pandemic coordinator, said that the pause would not generally interrupt the momentum of the country’s vaccination campaign.

    “In the very short term, we do expect some impact on daily averages as sites and appointments transition from Johnson & Johnson to Moderna and Pfizer vaccines,” he said. “We have more than enough Pfizer and Moderna vaccine supply to continue or even accelerate the current pace of vaccinations.”

    Noah Weiland and Madeleine Ngo contributed reporting.

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    Drug Overdose Deaths Have Surged During the Pandemic, C.D.C. Says

    On Tuesday, several dozen organizations that work on addiction and other health issues asked Mr. Biden’s health and human services secretary, Xavier Becerra, to “act with urgency” and eliminate the rule that doctors go through a day of training before getting federal permission to prescribe buprenorphine. Many addiction experts are also calling for abolishing rules that had already been relaxed during the pandemic so that patients don’t have to come to clinics or doctors’ offices for addiction medications.

    Although many programs offering treatment, naloxone and other services for drug users have reopened at least partly as the pandemic has dragged on, many others remain closed or severely curtailed, particularly if they operated on a shoestring budget to begin with.

    Sara Glick, an assistant professor of medicine at the University of Washington, said a survey of about 30 syringe exchange programs that she conducted last spring found that many closed temporarily early in the pandemic. After reopening, she said, many programs cut back services or the number of people they could help.

    “With health departments spending so much on Covid, some programs have really had to cut their budgets,” she said. “That can mean seeing fewer participants, or pausing their H.I.V. and hepatitis C testing.”

    At the same time, increases in H.I.V. cases have been reported in several areas of the country with heavy injection drug use, including two cities in West Virginia, Charleston and Huntington, and Boston. West Virginia’s legislature passed a law last week placing new restrictions on syringe exchange programs, which advocates of the programs said would force many to close.

    Mr. Biden’s American Rescue Plan Act includes $1.5 billion for the prevention and treatment of substance use disorders, as well as $30 million in funding for local services that benefit people with addiction, including syringe exchange programs. The latter is significant because while federal funds still largely cannot be spent on syringes for people who use drugs, the restriction does not apply to money from the stimulus package, according to the Office of Drug Control Policy. Last week, the administration announced that federal funding could now be used to buy rapid fentanyl test strips, which can be used to check whether drugs have been mixed or cut with fentanyl.

    Fentanyl or its analogues have increasingly been detected in counterfeit pills being sold illegally as prescription opioids or benzodiazepines — sedatives like Xanax that are used as anti-anxiety medications — and particularly in meth.

    Northeastern states that had been hit hardest by opioid deaths in recent years saw some of the smallest increases in deaths in the first half of the pandemic year, with the exception of Maine. The hardest-hit states included West Virginia and Kentucky, which have long ranked at the top in overdose deaths, but also western states like California and Arizona and southern ones like Louisiana, South Carolina and Tennessee.

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    Expert Panel to CDC to Vote on Johnson & Johnson Vaccine Pause

    An advisory committee for the Centers for Disease Control and Prevention will meet on Wednesday afternoon to discuss whether the Johnson & Johnson Covid-19 vaccine should remain paused while a possible link to rare blood clots is investigated.

    The emergency meeting follows the Food and Drug Administration’s announcement on Tuesday that it was studying six cases of rare and severe blood clots in women aged 18 to 48, one of whom died. All of the women had received the Johnson & Johnson vaccine before developing the clots, though it is unclear whether the vaccine is responsible. As of Tuesday, more than seven million people in the United States have received the shot, and another 10 million doses have been shipped out to the states, according to C.D.C. data.

    Following the call from federal health agencies, all 50 states, Washington, D.C., and Puerto Rico on Tuesday quickly paused or recommended that providers pause the administration of the vaccine. The U.S. military, federally run vaccination sites, and a host of private companies, including CVS, Walgreens, Rite Aid, Walmart and Publix also paused the injections.

    The Advisory Committee on Immunization Practices, or ACIP, is a panel of independent experts who advise the C.D.C. on its vaccine policies. At the meeting, the experts will be reviewing and debating the data from the six cases, and listening to comments from the public, before taking a vote on how to proceed. They could vote to recommend that the pause continues, for example, or to specify that it should apply only to a certain age or sex.

    reiterated on Wednesday that the pause in Johnson & Johnson vaccinations gives public health officials a chance to investigate the cases and discuss with health care professionals. He added that pauses are common when new vaccines and drugs are rolled out.

    “We’re just doing the due diligence we need to do to make sure everything is safe so we can continue with our vaccination efforts,” Dr. Murthy said on “CBS This Morning.”

    was not worth the trade-off of slowing the vaccination campaign and potentially eroding the public’s trust of vaccines in general.

    Madeleine Ngo contributed reporting.

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    Highlighting a rare potential vaccine side effect has risks of its own, public health officials warn.

    To federal health officials, asking states on Tuesday to suspend use of the Johnson & Johnson coronavirus vaccine until they could investigate six extremely rare but troubling cases of blood clots was an obvious and perhaps unavoidable move.

    But where scientists saw prudence, public health officials saw a delicate trade-off: The blood clotting so far appears to affect just one out of every million people injected with the vaccine, and it is not yet clear if the vaccine is the cause. If highlighting the clotting heightens vaccine hesitancy and helps conspiracy theorists, the “pause” could ultimately sicken — and even kill — more people than it saves.

    “It’s a messaging nightmare,” said Rachael Piltch-Loeb, an expert in health risk communications at the N.Y.U. School of Global Public Health. But officials had no other ethical option, she added. “To ignore it would be to seed the growing sentiment that public health officials are lying to the public.”

    The one-dose Johnson & Johnson vaccine was just beginning to gain traction among doctors and patients after its reputation took a hit from early clinical trials suggesting its protection against the coronavirus was not as strong as that from the vaccines made by Pfizer-BioNTech and Moderna. Before Tuesday’s pause, some patients were asking for it by name.

    But amid the blizzard of news and social media attention around the pause, those gains may well be lost, especially if the rare blood clotting feeds politically driven conspiracy theorists and naysayers, who seemed to be losing ground as the rate of vaccinations rose.

    The problem is explaining relative risk, said Rupali J. Limaye, who studies public health messaging at the Johns Hopkins Bloomberg School of Public Health. She noted that the potential rate of blood clotting in reaction to the vaccine is much smaller than the blood clotting rate for cigarette smokers or for women who use hormonal contraception, although the types of clots differ.

    And officials are not “pulling” the vaccine. They are simply asking for a timeout, in effect, to figure out how best to use it.

    Vaccinators were already fielding questions from worried patients on Tuesday.

    Maulik Joshi, the president and chief executive of Meritus Health in Hagerstown, Md., which has given 50,000 doses of all three vaccines without any reported major reactions, said he had a simple message to calm patients’ fears: “It’s a great thing that they have paused it, and this is science at work.”

    Jennifer Steinhauer, Madeleine Ngoand Hailey Fuchs contributed reporting.

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    F.D.A. Will Allow Abortion Pills by Mail During the Pandemic

    The Biden administration has decided to allow women to receive abortion pills by mail for the duration of the coronavirus pandemic, the latest development in an issue that has increasingly taken center stage in the American abortion debate.

    In a letter sent Monday to two leading organizations representing reproductive health physicians, the acting commissioner of the Food and Drug Administration said that the agency would temporarily stop enforcing its requirement that the first of two drugs needed to terminate an early pregnancy be dispensed in a medical clinic.

    The new policy counters a Supreme Court decision in January that sided with the Trump administration, which had appealed a federal judge’s decision last July to suspend the requirement. The judge had argued that the requirement put women at risk during the pandemic because they would need to visit clinics in person and often travel significant distances to do so.

    Abortion through medication, first approved by the F.D.A. in 2000, is increasingly becoming women’s preferred method for terminating a pregnancy. As of 2017, research estimated that about 60 percent of abortion patients early enough in pregnancy to be eligible — 10 weeks pregnant or less — chose medication abortion over suction or surgery.

    dispensed in clinics or hospitals by specially certified doctors or other medical providers. For years, reproductive health experts have urged that the requirement be lifted on the grounds that there are no significant safety reasons for in-person dispensing of a pill that women are then legally allowed to take on their own in any location, and that the restriction places the greatest burden on low-income women and those in areas with limited access to abortion providers.

    For several years, with the F.D.A.’s permission, researchers have been conducting a study that provides telemedicine consultations to women seeking abortions and mails them the pills. Their research has found the approach to be safe and effective.

    Additional data was collected in recent months from the experiences of women during the pandemic who received abortion pills by mail after the judge lifted the restriction and before the Supreme Court reinstated it.

    Dr. Janet Woodcock, the acting F.D.A. commissioner, wrote in her letter to the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine that studies of the pandemic experience “do not appear to show increases in serious safety concerns,” like bleeding, ectopic pregnancy or the need for surgical interventions “occurring with medical abortion as a result of modifying the in-person dispensing requirement during the Covid-19 pandemic.”

    Groups that oppose abortion objected to the decision. Jeanne Mancini, president of March for Life, said in a statement that allowing appointments for medication abortion via telemedicine posed “grave danger” to women’s safety, adding “chemical abortions should have more medical oversight not less.”

    letter in March to President Biden and Vice President Kamala Harris asking for the F.D.A. to lift the restrictions during the pandemic, welcomed the decision.

    “Mifepristone itself has demonstrated, through both clinical study and decades of use, to be a safe, effective medication,” the president and chief executive of the American College of Obstetricians and Gynecologists said in a statement. “Requiring the medicine to be dispensed in person, then taken elsewhere at the patients’ discretion, is arbitrary and does nothing to bolster the safety of an already-safe medicine.”

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    Worry Over 2 Covid Vaccines Deals Fresh Blow to Europe’s Inoculation Push

    BRUSSELS — First it was AstraZeneca. Now Johnson & Johnson.

    Last week, British regulators and the European Union’s medical agency said they had established a possible link between AstraZeneca’s Covid-19 vaccine and very rare, though sometimes fatal, blood clots.

    On Tuesday, Johnson & Johnson said it would pause the rollout of its vaccine in Europe and the United States over similar concerns, further compounding the continent’s one-step-forward-two-steps-back efforts to quickly get people immunized against the coronavirus.

    European officials had been confident that they had secured enough alternative vaccine doses to take up the slack of the AstraZeneca problems and achieve their goal of fully inoculating 70 percent of the European Union’s adult population — about 255 million people — by the end of the summer.

    On Tuesday, European officials did not immediately say whether they believed the milestone would also survive the Johnson & Johnson suspension. But the European commissioner for health, Stella Kyriakides, wrote on Twitter that “Today’s developments with the J&J vaccine in the US are under close monitoring” by the bloc’s medicines regulator.

    months of short supplies and logistical problems.

    There is mounting evidence that the concerns are eroding Europeans’ willingness to get the AstraZeneca vaccine in particular, and threatening to elevate already high levels of vaccine hesitancy generally.

    YouGov poll published last month, 61 percent of the French, 55 percent of Germans and 52 percent of Spaniards consider the AstraZeneca vaccine “unsafe.” That is in stark contrast to the findings of a similar poll from February, when more people in those countries, with the exception of France, believed that the shot was more safe than unsafe.

    Regulators have asked vaccine recipients and doctors to be on the lookout for certain symptoms, including severe and persistent headaches and tiny blood spots under the skin. Doctors’ groups have circulated guidance about how to treat the disorder.

    In Poland, where the vaccination campaign relies to a large extent on AstraZeneca and where its use has not been restricted, a recent poll showed that given a choice, fewer than 5 percent of Poles would choose the AstraZeneca shot.

    Almost everywhere across the European Union, it seems, many are eager for alternatives, as the new types of vaccines that include the Moderna and Pfizer, which utilize science known as “mRNA,” have not been associated with similar side effects.

    Data from the 27 E.U. member states by the European Center for Disease Prevention and Control shows that over all, 80 percent of vaccine doses distributed to the bloc have already been administered. That share drops to 65 percent for AstraZeneca, however, suggesting that many of its doses are sitting unused.

    Yet it is hard to predict how serious a blow the latest twist in the AstraZeneca saga — and the new Johnson & Johnson concerns — will be to E.U. vaccination efforts, as officials in Brussels have made big if belated efforts to turbocharge the second-quarter supply of doses.

    The European Union is poised to receive at least 300 million doses of various vaccines, three times what it got in the first quarter. Two hundred million are slated to come from Pfizer/BioNTech. Moderna is expected to deliver 35 million doses. Another 55 million doses are due of the Johnson & Johnson jab, and 70 million from AstraZeneca.

    In the rosiest scenario, the European Union could get up to 360 million doses by June.

    On Thursday, after Spain’s government changed the age threshold for the AstraZeneca shot, two-thirds of people called up for vaccination in Madrid did not show up, Antonio Zapatero, the regional health minister, told a news conference on Friday.

    He attributed the no-show by 18,200 people to “confusion” generated by Spain’s central government, which said on Wednesday that the AstraZeneca vaccine should be given only to people over 60. Before this change, Mr. Zapatero said, the rate of abstention was 2 percent.

    In Belgium, where the use of the AstraZeneca vaccine has also been limited, the authorities said they did not expect major delays in the overall rollout, but they are still concerned about the confusion that the rare blood clotting issue is causing.

    Yves Van Laethem, a top epidemiologist who is the country’s Covid task force spokesman, said he expected a two-week delay that would mostly affect younger age groups in late summer. He said the E.U. regulator guidance had only partly helped in clarifying the situation.

    The European Medicines Agency’s opinion “wasn’t very clear, and it is also part of the problem,” Dr. Van Laethem said in an interview. “When you say, ‘We don’t apply limitations, but we just say there are serious side effects,’ there is part science and part diplomacy in that.”

    He said the limited effect that the new AstraZeneca issues would have on Belgian’s rollout was in large part because the country had ordered big shares of other vaccines.

    Although all E.U. countries have been offered a chunk of each vaccine approved in the bloc so far — AstraZeneca, Johnson & Johnson, Moderna and Pfizer — many opted to forgo parts of their share of more expensive or cumbersome vaccines like Pfizer and Moderna early on, instead favoring the AstraZeneca jab.

    “In Britain or Eastern Europe, a big part of the campaigns are based on AstraZeneca,” Dr. Van Laethem said.

    Wealthier bloc members like Denmark, France, Germany and the Netherlands can better compensate for the loss of confidence in AstraZeneca, because they acquired extra doses of other vaccines — especially Pfizer — through a secondary market after poorer E.U. nations gave theirs up.

    Those countries — including Bulgaria, Croatia, Latvia and Slovakia — are likely to be less able to quickly offer alternatives.

    Dr. Van Laethem, the Belgian immunologist, said that the national and European authorities needed to better communicate the costs and benefits of taking the AstraZeneca dose versus and the other authorized vaccines.

    Experts worry that even limited concerns over one vaccine’s unlikely side effects can affect people’s overall willingness to be immunized.

    “The main thing is to make people understand that the problem is the virus,” he said. “We have to vaccinate people — the risk linked to the virus is higher than those rare side effects.”

    Raphael Minder contributed reporting from Madrid and Constant Méheut from Paris.

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    How Worried Should You Be About the J&J Vaccine and Blood Clots?

    On Tuesday morning, U.S. federal health regulators recommended a pause in the use of Johnson & Johnson’s Covid-19 vaccine while they investigated six reports of blood clots in women ages 18 to 48. One has died, and a second is hospitalized in critical condition.

    As of Monday, 6.8 million people in the United States had received the vaccine without any other serious adverse reactions reported.

    Experts have yet to determine to what extent, if any, the vaccine is responsible for the clots. But the investigation follows actions by European regulators who concluded that a vaccine made by AstraZeneca may also be the cause of a similar, extremely rare clotting disorder.

    U.S. and European public health experts have emphasized that for most people, the benefits of the Covid vaccines far outweigh the risks.

    recommends that people who have received the Johnson & Johnson vaccine within the past three weeks should contact their doctors if they experience severe headaches, abdominal pain, leg pain or shortness of breath. People should not be concerned about mild headaches and flu-like symptoms in the first few days after vaccination. Those are common, harmless side effects brought on by the immune system’s production of a defense against the coronavirus.

    During clinical trials and after vaccines go into wide use, experts keep track of any medical problems experienced by people who receive them. If an unusually large cluster of cases turns up, regulators may decide to pause a trial or stop the use of a vaccine to investigate further.

    Pauses are common, and typically the investigations reveal that the medical problems were a matter of coincidence. If the investigation reveals that a vaccine does pose a risk, regulators may write new guidance about who should or should not receive it.

    regulators have said, roughly one in 1,000 people are affected by a blood clot in a vein every year.

    But the clotting disorder of concern in the vaccine recipients is much rarer and different from typical blood clots. In addition to clotting in the brain — called cerebral venous sinus thrombosis, or CVST for short — the patients all had a notably low level of platelets, which left them prone to abnormal bleeding.

    said the company was aware of an extremely rare disorder involving people with blood clots in combination with low platelets in a small number of individuals who have received our COVID-19 vaccine. “In addition, we have been reviewing these cases with European health authorities,” the company said in its statement. “We have made the decision to proactively delay the rollout of our vaccine in Europe.”

    At the news conference on Tuesday, Dr. Marks of the F.D.A. said the cases were “very, very similar.”

    a number of possible symptoms, including swelling in the leg, persistent abdominal pain, severe and persistent headaches or blurred vision, and tiny blood spots under the skin beyond the area where the injection was given.

    But that set of symptoms was so vague that almost immediately, British emergency rooms experienced a surge in patients who were worried that they fit the description.

    Nonetheless, German researchers say that such symptoms in vaccine recipients must be followed up. Blood tests can detect the antibodies.

    Doctors in Germany and Norway have treated patients with blood-thinning drugs to try to stop the growth of the clots, and with intravenous immune globulin, which can help eliminate the misguided antibodies that are causing the problem.

    AstraZeneca vaccines. One recipient, a physician in Florida, died from a brain hemorrhage when his platelet levels could not be restored, and others have been hospitalized. U.S. health officials have said that the cases are being investigated, but they have not reported the findings of those reviews and have yet to indicate that there is any link to the vaccines.

    Benjamin Mueller contributed reporting.

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