although the vaccine remains powerfully effective against severe disease and death.

Experts were divided on the utility of booster shots so soon after vaccination began. Experience with other diseases indicates that older people and those with weak immune systems might benefit, but there is little hard evidence with the coronavirus.

“The problem here is, we’re just sort of going on immunological priors, rather than really great data to justify things one way or the other,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “I totally understand the decision, but I think we have to acknowledge that there’s a wide range of uncertainty on what it’s going to do.”

Booster doses may help some people with weak immune systems, but others may show little improvement even after a third dose, and still others may not need a booster at all, scientists say.

While dozens of mostly wealthy countries, including the United States and most of Europe, have administered more than 100 doses per 100 people, many other nations remain below five per 100 — primarily in Africa, where cases have soared as the Delta variant spreads.

Doctors Without Borders said recently that it would be “unconscionable” to give booster doses in richer nations before people in poorer ones get their first doses.

“Wealthy governments shouldn’t be prioritizing giving third doses when much of the developing world hasn’t even yet had the chance to get their first Covid-19 shots,” Kate Elder, the senior vaccines policy adviser at Doctors Without Borders’ Access Campaign, said in a statement.

a so-called vector vaccine, like AstraZeneca or Johnson & Johnson.

It is the latest sign that governments are encouraging their citizens to mix and match vaccines in the hope of provoking a more protective immune response against Covid-19. Early results from a British vaccine study showed that volunteers produced high levels of antibodies and immune cells after getting one dose each of the Pfizer-BioNTech and AstraZeneca-Oxford shots.

The new German guidelines announced Monday also went a step further in encouraging parents to vaccinate children between 12 and 17, announcing that doctors and vaccination centers across the country would make the jab available to them before the start of the new school year.

Health ministers stopped short of making a formal recommendation for vaccinating children, but the move made plain their impatience with Germany’s Standing Committee on Vaccinations, which has so far refrained from guiding parents one way or the other, pending more data becoming available.

Vaccinating children “is one building block to allow a safe start into the new school year after the summer vacation,” Mr. Holetschek said.

Apoorva Mandavilli contributed reporting from New York, Benjamin Mueller from London, Aurelien Breeden from Paris, Gaia Pianigiani from Rome, Monika Pronczuk from Brussels, Raphael Minder from Madrid and Thomas Erdbrink from Amsterdam.

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