But some are not convinced. As many, like Gallagher, call for another dose, the city and county of San Francisco announced that anyone who received the Johnson & Johnson one-dose vaccine may get a booster shot of either the Pfizer or Moderna vaccine. On the other side of the issue is the World Health Organization, which this week said no one should get booster shots until everyone has access to their first doses. 

So far, high-income countries have given about 100 vaccine doses for every 100 people, while low-income countries have given just 1.5 doses for every 100 people.

“Which means, in some of the most vulnerable countries in the world with the weakest health systems, health care workers are working without protection … the older populations remain at high risk,” said Bruce Aylward, MD, the WHO’s senior adviser on organizational change.

Many scientists do believe that boosters can help some people. Kuritzkes notes those who are immunocompromised, and mounted a weak initial immune response to the vaccine, are likely to benefit from a booster. The Advisory Committee on Immunization Practices (ACIP) came to the same conclusion in July, but the group does not have regulatory authority and can only make recommendations.

Otto Yang, MD, a professor of medicine and associate chief of infectious diseases at UCLA Health, says the data so far is “pointing very strongly” in favor of boosters — particularly given what scientists know about other types of coronaviruses. But, he says, it depends on how you define “protection.”

For example, there is protection from symptomatic infection, and then there is protection from serious illness and death. Generally, antibodies are responsible for the first, and T cells are responsible for the latter. Because T cells tend to last longer than antibodies, Yang says it is likely that the vaccine will protect against serious illness longer than symptomatic infection.

“It’s really hard to say when people will need them. I’m guessing somewhere in the range of 6 months to a year of when they get their shot,” he says. But, he says, though there is no reason to think a booster would be unsafe, there is still no data on that. “Right now, that’s venturing into the unknown.”