The Pfizer-BioNTech Covid-19 vaccine's protection against coronavirus infection wanes over time, according to a new study published in The Lancet, but the vaccine remains 90% effective against severe disease that causes hospitalization after six months.

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What the new study reveals about how the vaccine's protection wanes

For the study, researchers looked at the medical records of 3.4 million patients ages 12 and older enrolled in Kaiser Permanente Southern California's health care system between Dec. 14, 2020 and Aug. 8, 2021.

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More than 1 million of those individuals received the Pfizer-BioNTech vaccine during the period studied. Researchers compared rates of Covid-19 infection and hospitalization in these individuals with those of 2.3 million unvaccinated patients.

The researchers found that, during the period studied, the vaccine was 73% effective against infection and 90% effective at preventing hospitalizations. However, the researchers found that the vaccine's effectiveness against infection waned over time, dropping from 88% in the first month after vaccination to 47% in the fifth month.

However, effectiveness against hospitalizations held at around 90% throughout the period studied across all age groups, the researchers found.

Those hospitalized with Covid-19 during the study period were disproportionately "older, male, had comorbidities, and had greater previous health care utilization than those not admitted to the hospital," the researchers found.

The researchers also explored the drivers of declining vaccine effectiveness. Previously, some infectious disease experts had suggested the decline might be due to the emergence of the highly infectious delta variant of the coronavirus. The researchers determined, however, that a recent shot of the Pfizer-BioNTech vaccine was just as protective against infection with delta as with other variants. Specifically, the researchers found the vaccine was 93% effective at protecting against the delta variant after one month, with overall effectiveness dropping to 53% after four months. The vaccine remained 93% effective at preventing hospitalization from the delta variant at six months.

As such, the "[r]eduction in vaccine effectiveness against [coronavirus] infections over time is probably primarily due to waning immunity with time rather than the delta variant escaping vaccine protection," the researchers concluded. (Healy, Los Angeles Times, 10/4; Walker, MedPage Today, 10/4; Choi, The Hill, 10/4)