Immunity against infection from COVID-19 vaccines is waning over time, a new study suggests.
Researchers from the Leumit Health Services in Israel and the U.S. National Institutes of Health (NIH) looked at people inoculated with the Pfizer-BioNTech vaccine.
They found that, 90 days after receiving the second dose, the risk of contracting the virus more than doubles and only keeps increasing as far as six months out.
What's more, the percentage of tests coming back positive for COVID-19 quadrupled from 2.4 percent at least 90 days later to 10.6 percent at least 150 days later.
The finding add more evidence that, while the Pfizer vaccines provides strong protection in the initial weeks, booster shots are likely needed to sustain long-term protection from Covid.
Researchers looked at more than 80,000 adults who were fully vaccinated with the Pfizer-BioNTech vaccine. Pictured: Yan Shaposhnik, 10, receives a Pfizer-BioNTech COVID-19 vaccine in Netanya, Israel, November 23
Far right graph: The percentage of tests that came back positive for Covid increased from 1.3% prior to 90 days (purple) to of 2.4% 90 to 119 days later (yellow), to 10.3% 150 to 179 days later (blue) across all age groups
Israel was one of the first countries to roll out use of Pfizer's Covid vaccine in December 2020 and the U.S. was not far behind.
The Middle Eastern country experienced a resurgence in cases in September while infections are creeping up now in the U.S.
This has led many experts to fear that immunity from the standard two-dose regimen wanes over time.
Researchers say determining the risk of infection as more time elapses since dose two can lead to important information about the optimal timing of a third dose.
For the study, published in The BMJ, the team looked at the electronic health records of 80,057 adults in Israel.
All of the adults were fully vaccinated with the Pfizer shot and had no history of a prior COVID-19 infection.
Of this group, 7,973 - or 9.6 percent - ended up testing positive for COVID-19.
Risk of infection was 2.37-fold higher between 90 and 119 days after completing the vaccine series compared to the initial 90 days. Pictured: A healthcare worker administers a dose of the Pfizer-BioNTech COVID-19 vaccine in Key Biscayne, Florida, August 2021
After matching patients to controls that were of the same age and racial/ethnic background, researchers found the rate of positive Covid tests increased the more time had passed since the second dose.
Between 21 and 89 days after the second dose, only 1.3 percent of tests came back positive.
However, between 90 and 119 days later, this rate nearly doubled to 2.4 percent, and only continued to grow.
A total of 4.6 percent of tests came back positive 120 to 149 days after the second dose, increasing to 10.3 percent 150 to 179 days later and to 18.5 percent 180 days or more later.
Next, researchers looked at the risk of infection across all age groups compared to 90 days after the second dose.
They found that the risk of infection with 2.37-fold higher between 90 and 119 days after completing the vaccine series.
This only continued to increase to 2.66-fold higher after 120 to 149 days; 2.82-fold higher after 150 to 179 days; and 2.82-fold higher after 180 days or beyond.
The researchers say there could be other factors influencing the increased risk and test positivity rate including household size and Covid variants.
However, they believe that a third dose of Pfizer's vaccine should be considered, if not administered to all adults.
'In this retrospective large cohort study, performed in individuals who received two doses of the BNT162b2 mRNA vaccine, protection seemed to decrease over time, and the risk of breakthrough infection by SARS-CoV-2 increased progressively compared with the protection provided during the initial 90 days,' the authors wrote.
'Interpretation of the findings of the study is limited by the observational design, but the results suggest that consideration of a third vaccine dose might be warranted.'
Last week, the U.S. Food and Drug Administration approved Pfizer's COVID-19 vaccine booster to all fully vaccinated adults aged 18 and older six months after the inital regimen.
The agency is also allowing Americans to mix and match their shots, meaning someone fully vaccinated with the Moderna or Johnson & Johnson vaccines can receive a Pfizer booster.
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