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When Pfizer CEO Albert Bourla said people will in 6 to 12 months, and that annual vaccines were possible, public health and infectious disease experts were quick to put on the brakes.

There aren't enough data yet to make that call, said Paul Offit, MD, a vaccine expert at Children's Hospital of Philadelphia.

"We'll have a better idea in about 1 year," Offit told 口袋女优.

Experts have said they do believe it's likely that boosters will eventually be necessary. When that will be, how often they'll be needed, and whether that will vary by vaccine brand, or by differences in immune response, is still anyone's guess.

"I fully believe we are going to need boosters," said Robert Schooley, MD, an infectious disease expert at the University of California San Diego. "It may be that we need to vaccinate more frequently if, over time, we have a less vigorous immune response. It may be that vaccines with potent immunity like the mRNA vaccines will need to be boosted less frequently than other vaccines using other platforms."

"It may be that how soon we need to be revaccinated depends on how the virus evolves in terms of its ability to evade current vaccines," he added. "These are all unknowns right now."

Why is Schooley so certain that boosters will be a necessity? For one, breakthrough infections, while rare, have occurred after vaccination. Reinfections also have occurred, even though they appear to be less common -- though their true rate remains unknown. The four other coronaviruses that regularly circulate through the population are able to reinfect people, so "there's no reason to expect this will be different," Schooley said.

While two other human coronaviruses, MERS and the first SARS, have shown a long-standing immune response in lab studies, these "don't show if when you're re-exposed, whether you get sick," he said. "That's where the rubber hits the road."

"Measuring something in the lab is reassuring but not definitive," he added. "We don't know for sure about SARS and MERS because these viruses aren't circulating and people aren't regularly re-exposed."

In addition, leaving large swaths of the globe unvaccinated is sure to breed new variants that could evade vaccines, he said. "If we continue to let this virus replicate in large pockets of the world, we can expect it to come back in an evolved form and require revaccination."

Given all the uncertainty, Schooley said researchers are closely monitoring several factors to better judge the need for boosters. This includes coming to a better understanding of immune responses following vaccination.

"We don't yet know what parameters are most important in reinfection," he said. "We study neutralizing antibodies because they're easy, but it may be that cell-mediated responses are more important. We don't completely understand the lab parameters that are key in telling us who needs to be revaccinated."

The other element is monitoring reinfections, he said. "As we get a better idea about when reinfections occur and whether they're accelerating in terms of frequency, we can do a mathematical model and figure out the risk/benefit ratio for going ahead with a booster."

Schooley said monitoring systems will likely reveal an uptick in mild disease as immunity begins to wane. "We won't see people in the ICU and dying," he said. "When the tempo of symptomatic but not severe illness picks up, we can get ahead of that with vaccination."

That's why it's so critical to have boosters at the ready, he said. "We can nip it in the bud with a vaccine that's been tweaked to focus on the variants circulating at the time."

Vaccine makers are already preparing for that likelihood. the efficacy of a third dose of its vaccine given 6 to 12 months after the initial two doses, as well as a new version tailored to the B.1.351, or "South African," variant. that is testing a third dose and an updated shot targeted to B.1.351.

So far, both vaccines have reported good efficacy at the 6-month mark, at and , with higher rates for protecting against severe illness and death. But that's no reason to relax, Schooley said.

"The important thing is to be vigilant and act on what we see," he said, "rather than stumbling into revaccination."

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