How long does protection against COVID-19 vaccines last?
Experts don’t know yet, as they are still studying vaccinated people to see when the protection may drop. How well the vaccines work against emerging variants will also determine if, when and how often additional injections are needed.
“We only have information as long as the vaccines have been studied,” said Deborah Fuller, a vaccine researcher at the University of Washington. “We have to study the vaccinated population and start to see, at what point do people become vulnerable to the virus again?”
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So far, Pfizer’s ongoing trial indicates that the company’s two-dose vaccine remains highly effective for at least six months, and probably longer. People who received Moderna’s vaccine also still had remarkable levels of virus-fighting antibodies six months after the second required injection.
Antibodies don’t tell the whole story either. To fight off invaders like viruses, our immune system also has another line of defense called the B and T cells, some of which can linger long after antibody levels drop. If they encounter the same virus in the future, those battle-tested cells may be able to spring into action more quickly.
Even if they don’t completely prevent disease, they can help to mitigate its severity. But it is not yet known exactly what role such “memory cells” can play in the coronavirus – and for how long.
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While current COVID-19 vaccines probably last for at least about a year, they probably don’t provide lifelong protection like measles injections, said Dr. Kathleen Neuzil, a vaccine expert at the University of Maryland.
“It will be somewhere in the middle of that very wide range,” she said.
Variants are another reason we may need an extra shot.
Current vaccines are designed to work against a particular peak protein on the coronavirus, said Mehul Suthar of the Emory Vaccine Center. If the virus mutates enough over time, vaccines may need to be updated to increase their effectiveness.
So far, the vaccines appear to be protective against the remarkable variants that have emerged, although slightly less against the variants first discovered in South Africa.
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If it turns out that we need another injection, a single dose may extend the protection of the current injections or include vaccination for one or more variants.
The need for follow-up shots will also depend in part on the success of the global vaccination pressure and the slowing of the transmission of the virus and emerging variants.