When do vaccines wear off

There are several ways to do this: one is to look at whether people who have been vaccinated become infected with Covid, and another is to look at levels of different components of the immune system in vaccinated individuals and see how these change over time. But what is behind the apparent decline and what it means in practice, and for booster shots, is less clear. But as experts have pointed out, such findings are probably down to many factors , not simply that protection afforded by vaccination is wearing off.

Covid vaccines are somewhat less effective against the Delta variant than the Alpha variant. Delta is also more transmissible, which — together with policy changes such as relaxation of restrictions — means the chances of someone being exposed to the virus has risen.

Another factor is that the Zoe survey is based on a self-selecting sample of app users. Pfizer reported on its efficacy in a preprint that has not been reviewed by outside scientists; Moderna released a company statement.

A report in The New England Journal of Medicine NEJM in April showed that 33 participants who had received the Moderna vaccine during the Phase I trial had a gradual decline in antibody protection—and, based on the slope, Iwasaki says, that is hopeful news. It should be noted that the trial occurred prior to Delta becoming the predominant virus variant. Another measure is T cells, which scientists are still studying for their ability to kill virus-infected cells in the context of COVID, and which may also provide important protection.

T cells are more difficult to measure, Iwasaki says. But they may be important—last year, a study in Nature showed that people who were infected with severe acute respiratory syndrome SARS , a different coronavirus outbreak that killed almost people in , maintained T-cell immunity 17 years after they recovered.

Still another way to predict how long protection might last is by looking at natural immunity, says Dr. No one knows for sure whether one vaccine will last longer than another.

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