COVID-19 vaccines offer strong protection against severe disease and death, but breakthrough infections — cases that occur two or more weeks after complete vaccination — are possible.
Now, a new study adds to a growing body of evidence that shows the risk of infection gradually increases after your second shot of a two-dose COVID-19 vaccine, suggesting booster shots may play a critical role in the ongoing pandemic.
An analysis of more than 80,000 electronic health records of adults who took a PCR coronavirus test at least three weeks after receiving their second dose of the Pfizer vaccine found the rate of positive results increased as more time passed since a second shot.
Of the total number of adults included in the study, 7,973 people tested positive between May and September.
Across all age groups, 1.3% of people tested positive about a month to three months after their second dose, 2.4% after about three to four months, 4.6% after about four to five months, 10.3% after about five to six months and 15.5% after six months.
Study participants were recruited from the Research Institute of Leumit Health Services in Israel, were an average of 44 years old, had not received a booster shot and had no prior coronavirus infection, according to the study published Thursday, Nov. 25 in The BMJ.
Researchers admit they cannot “rule out the possibility” that other factors such as household size, coronavirus strain or population density could have influenced people’s chances of contracting a breakthrough case.
But the team says its findings reveal that protection from COVID-19 vaccines wanes over time and that booster shots “might be warranted.”
A separate study published in October of more than 3 million people found the Pfizer vaccine’s effectiveness against coronavirus infection dropped from 88% during the first month after complete vaccination to 47% after five months. A similar decline was found among delta variant infections.
Yet, as is the case with all available COVID-19 vaccines, protection against severe illness remains strong.
The Pfizer vaccine was 93% effective against hospitalizations caused by the delta variant for all ages up to six months after complete vaccination, according to the study.
“After you are vaccinated, there is a burst of protection. Over time, there is the potential for immunity to wane — however, it is important to remember that you are still protected,” Dr. Eric Ascher, a family medicine physician at Lenox Hill Hospital in New York, told Healthline. “You have many different cells in your body that are still activated long after vaccination that allow you to be protected against severe virus.”
Any adult can receive a booster shot at least six months after they received their second dose of the Pfizer or Moderna vaccines or at least two months after they received their single dose of the Johnson & Johnson shot.
About 37.5 million people in the U.S. have received a COVID-19 booster shot as of Nov. 24, according to a Centers for Disease Control and Prevention tracker.