Scientists are raising serious questions about the wisdom of the push for Covid-19 boosters unveiled on Wednesday by President Joe Biden. It’ll be hard to judge whether extra shots for the vaccinated will be helpful or counterproductive until more of the data behind the decision become available. The officials announcing the decision to give shots to those vaccinated eight months earlier said it was justified by data on an increase in mild infections and speculation that this might evolve into something worse. But it’s not clear to some scientists that severe disease and death will indeed rise among the vaccinated even if there are more infections. And outside scientists havent had a chance to analyze the data that the government used to infer the need for boosters. Additionally, some public health specialists dispute the ethics of the decision given the pressing need for doses elsewhere in the world. In contrast, there isn’t much controversy over a recommendation by the Centers for Disease Control that an extra shot is merited for a small number of vaccinated people with weakened immune systems — cancer patients and people who have received organ transplants, for example. Wednesday’s general booster decision is different. It rests on data coming out of the US and Israel showing that breakthrough infections among vaccinated people are on the rise. That fact has been interpreted to mean that the immunity conferred by vaccines is waning. But it could just as easily indicate nothing more than an inevitable rise attributable to the more infectious delta variant, which is now dominant but which hadn’t been detected when the first rounds of vaccines were administered. It’s also risky to infer waning immunity by comparing those who got the vaccine early to those who got it later, because the first shots went to the oldest, sickest people. Meanwhile, the World Health Organization and other global health bodies are arguing that the shots are more likely to save lives if given away to countries where health-care workers and the elderly still haven’t had access to vaccines at all. Eric Toner, a senior scholar at the Johns Hopkins Bloomberg School of Public Health, said he’s confident that federal officials based their decision on data — he just hasn’t been able to see or analyze it yet. Neither has infectious disease expert Monica Gandhi of the University of California at San Francisco, but based on her understanding of the immune system and the way vaccines work, she’s not convinced that protection against severe disease or death will wane. She said that first doses of vaccines trigger a temporary surge of antibodies that normally wanes over a period of weeks. Human blood would be thick as glue if it carried full antibodies to all the diseases that people are immunized against, she said. The lasting protection comes not from the antibodies themselves but from T cells and memory B cells, which detect and kill infected cells and create new antibodies. Those cells can last for years or decades. They don’t always protect against mild disease because it takes a few days for them to become activated, but they should prevent most people from suffering from the system-wide invasion that’s characterized severe illness. Gandhi posed another critical science question about the wisdom of an extra shot. All that are available now, she said, are shots created to fight the original variant, not the currently dominant delta. But human immune systems can make modified antibodies geared to attack the new variant. There’s some flexibility in the new antibodies produced by the memory B cells, which hide away in lymph nodes and bone marrow after being activated by the initial shots. It’s not clear to her whether vaccinated people would be better off if given a booster dose that triggers production of antibodies geared to the old version of the coronavirus than they would be by letting the existing B cells go to work against the new one. Wednesday’s announcement also left open questions about the differences between the three vaccines approved for use in the US. There’s some evidence that for two-shot vaccines, the ones made by Pfizer-BioNTech and Moderna, antibodies last longer if there’s a longer interval between shots. (The Johnson & Johnson vaccine requires only one shot.) University of Pennsylvania vaccine expert Stanley Plotkin argued in a paper published in January that two-shot Covid-19 vaccines would work at least as well with longer intervals and could save more lives. The relatively short intervals recommended by the manufacturers were not optimized for effectiveness, but were chosen in part to get the vaccines out and approved faster. The UK, which relied heavily on a different two-shot vaccine made by AstraZeneca, went against manufacturer instructions and asked people to wait a full 12 weeks between shots. Some scientists applauded that choice, and the country’s delta wave crested and started declining a month ago. The booster plan will raise many ethical discussions about individual choices and national ones. There’s a practical consideration that getting more shots out in the world could minimize the chances of a new variant cropping up. But these are not decisions that can be made based on science alone.