Study shows Pfizer COVID-19 vaccine effectiveness declines after six months

A study published on Monday in The Lancet medical journal found that the efficacy of the Pfizer-BioNTech COVID-19 vaccine fell below 50 percent after about six months after the second dose.

The Pfizer-funded study found that Pfizer's vaccine was 88 percent effective in the first month after full vaccination but dropped to 47 percent effectiveness at about six months. The vaccine was also found to be highly effective against the delta variant, which was found to be over 90 percent effective in the first months before dropping to 53 percent effectiveness after four months.

Researchers determined that the waning immunity had to do with the amount of time since an individual was given the second shot rather than due to the highly infectious delta strain.

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"Our results provide support for high effectiveness of [Pfizer-BioNTech COVID-19 vaccine] against hospital admissions up until around 6 months after being fully vaccinated, even in the face of widespread dissemination of the delta variant," the researchers wrote. "Reduction in vaccine effectiveness against SARS-CoV-2 infections over time is probably primarily due to waning immunity with time rather than the delta variant escaping vaccine protection."

Protection against hospital admission remained high throughout, being 93 percent effective up to six months after administration.

For the study, researchers looked at the electronic records of the Kaiser Permanente Southern California (KPSC) health care system, looking at all the system's patients ages 12 and up. Researchers looked at 3.4 million people in the KPSC health care system that they studied from December 2020 to last August.

Researchers determined that individuals fully vaccinated with the Pfizer-BioNTech vaccine had an overall 73 percent effective protection against COVID-19 infection and a 90 percent effective protection against COVID-19 related hospitalization.

"Our results reiterate in a real-world US setting that vaccination with [the Pfizer-BioNTech COVID-19 vaccine] remains an essential tool for preventing COVID-19, especially COVID-19-associated hospital admissions, caused by all current variants of concern," they wrote.

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The results of these studies reiterate findings from Pfizer and BioNTech that were released in July. The preprint study found that the vaccine's effectiveness reached a peak of 96.2 percent within two months after the second dose, with the shot's effectiveness found at the time to decrease by about 6 percentage points every two months afterwards.

The results of this study come out just weeks after the Food and Drug Administration approved a booster dose of the Pfizer coronavirus vaccine for the elderly and those at high risk of infection, with the decision made in light of earlier data that suggested the vaccine efficacy fell after some months.

"We believe boosters have an important role to play in addressing the continued threat of this disease, alongside efforts to increase global access and uptake among the unvaccinated," Pfizer CEO Albert Bourla said at the time.