COVID-19 vaccine protection wanes with time, strengthening case for boosters

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Protection against COVID-19 infection following inoculation from the Pfizer/BioNTech vaccine begins to gradually wane over time, particularly 90 days after receiving the second dose, strengthening the case for booster shots, according to new findings published in The BMJ.

The study was carried out by the Research Institute of Leumit Health Services in Israel. Israel was one of the first countries to roll out a large-scale COVID-19 vaccination campaign in December 2020, though it has seen a resurgence of infections since June 2021. 

The findings confirm that the Pfizer-BioNTech vaccine provided excellent protection in the initial weeks after vaccination, but suggest that protection wanes for some individuals with time.


Across the world, large-scale COVID-19 vaccination campaigns are helping to control the spread of the virus. But even in countries with high vaccination rates, breakthrough infections can occur, which scientists think is due to a gradual loss of immunity over time.

Examining the time elapsed since vaccination and risk of infection could provide important clues about the need for a third injection and its preferred timing.

To do this, the researchers examined electronic health records for 80,057 adults (with an average age of 44) who received a PCR test at least three weeks after their second injection, and had no evidence of previous COVID-19 infection.

Of these participants, 7,973 (9.6%) had a positive test result. They were then matched to negative controls of the same age and ethnic group who were tested in the same week. 

The rate of positive results increased with the time elapsed since a second dose. For example, across all age groups, 1.3% of participants tested positive 21-89 days after a second dose, but this increased to 2.4% after 90-119 days; 4.6% after 120-149 days; 10.3% after 150-179 days; and 15.5% after 180 days or more.

And after taking into account other potentially influential factors, the researchers found a significantly increased risk of infection with time elapsed since a second dose. 

Compared with the initial 90 days after a second dose, the risk of infection across all age groups was 2.37-fold higher after 90-119 days; 2.66-fold higher after 120-149 days; 2.82-fold higher after 150-179 days; and 2.82-fold higher after 180 days or more.

Other factors such as household size, population density and virus strain were not factored into the study. But researchers are confident about the findings, and suggest that a booster dose is likely appropriate.


The U.S. Food and Drug Administration authorized booster shots of the Moderna and Pfizer COVID-19 vaccines for all adults ages 18 and older earlier this month.

The Centers for Disease Control and Prevention Advisory Committee on Immunization Practices has recommended the booster shots for all adults, which means those newly eligible should be able to get a booster vaccine before the end of the year. The CDC recommends that adults should get a booster at least six months after their last COVID-19 vaccine. Individuals should contact the vaccination site where they got their original shots, check their local pharmacy for appointments, or contact their local or state health departments to find a location.

Since Moderna and Pfizer-BioNTech initially submitted data on their boosters, the FDA said it has additional real-world data on the recently increasing number of cases of COVID-19 in the United States and on the risk of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the outer lining of the heart) following vaccination. 

The additional data has enabled the FDA to reassess the benefits and risks of the use of these vaccines in the general adult population. 

The FDA has determined that the benefits of a single booster dose of either the Moderna or Pfizer-BioNTech COVID-19 vaccines outweigh the risks of myocarditis and pericarditis and that the booster provides continued protection against COVID-19, including hospitalization and death.

Both Pfizer and Moderna are conducting post-authorization/post-marketing studies to assess known serious risks of myocarditis and pericarditis, according to the FDA.

The most commonly reported side effects by individuals who received a booster dose of the vaccines were pain, redness and swelling at the injection site, fatigue, headache, muscle or joint pain, and chills. Of note, swollen lymph nodes in the underarm were observed more frequently following the booster dose than after the primary two-dose series.

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