Pfizer said July 28 that its COVID-19 shot was 97 percent effective against severe disease and 83.7 percent effective against infection six months after the second dose.
In the study, published on the preprint server medRxiv but not yet peer-reviewed or published, scientists from Pfizer and its partner BioNTech found that the vaccine was 96 percent effective against symptomatic COVID-19 for the first two months after the second dose, and that number declined by about 6 percent every two months.
But the shot remained highly effective against severe disease.
"This drop is very slight — I wouldn’t say it's waning," Akiko Iwasaki, PhD, an immunologist at Yale School of Medicine in New Haven, Conn., told The New York Times. She added that she did not see in the new study any indication that booster shots are needed at this point. "These data don't support a need for that right now," she told the Times.
The study includes data from 42,000 people in six countries. The clinical trial, which started last July and ended in March, followed the volunteers for six months after vaccination. The study period ended before the spread of the delta variant, which is now the dominant strain in the U.S. and has caused COVID-19 cases to spike.
Find the full study results here.