As millions of eligible people consider getting a booster shot, many are wondering what the data tells us about the effectiveness of the vaccines, and how much their protection might be waning.

The good news is that a growing body of research shows that the vaccines authorized remain highly protective against severe disease and hospitalization — even against the Delta variant. There some exceptions among older people and those with weakened immune systems.

But while vaccines remain effective against the worst outcomes, a number of published studies show that their protection against infections has fallen.

As seen above, a study in England examined the vaccines’ effectiveness against the Delta variant over time. It found that the Pfizer vaccine was about 90 percent effective at preventing symptomatic infection two weeks after the second dose, but that the vaccine dropped to 70 percent effectiveness after five months.

The same study found that the Moderna vaccine’s protection also dropped over time.

Two additional studies, in the U.S. and in Canada, also found that the vaccines’ protection dropped over time.

Still, both the English and Canadian studies found that even after several months, the Pfizer and Moderna vaccines remained highly effective at preventing hospitalization.

“The main objective of the Covid vaccine is to prevent severe disease and death, and they are still doing a good job at that,” said Melissa Higdon of the Johns Hopkins Bloomberg School of Public Health, who compiles research on Covid vaccine performance.

But the decline in protection against infection will still have an impact. With true declines in vaccine effectiveness, we’ll likely see more cases overall,” Higdon said.

“It’s easy with all the discussion about boosters to lose that really important message that the vaccines are still working,” said Eli Rosenberg, the deputy director for science in the Office of Public Health at the New York State Department of Health. “Going from an unvaccinated to a vaccinated person is still the critical step.”

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