Two studies from the Centers for Disease Control and Prevention show that for the fully vaccinated, immunity to the COVID-19 virus is declining.
According to the CDC studies, the vaccine’s effectiveness, which was at 91% before the delta variant became the dominant variant of the virus, has dropped to 66%.
One of the studies released Tuesday showed that vaccine effectiveness decreased among health care workers who were fully vaccinated over the period when the delta variant became dominant.
According to researchers, the declining effectiveness of the vaccine could be due to a natural drop in the vaccines’ ability to fight off the virus, the fact that the delta variant is highly contagious, or some other factor.
“What we were trying to figure out is: Is this Delta, or is this waning effectiveness?” Ashley Fowlkes, an epidemiologist on the Covid-19 response team at the Centers for Disease Control and Prevention, and the study’s lead author told the New York Times. “Our conclusion is that we can’t really tell.”
The CDC added that the decline should be “interpreted with caution” because the waning effectiveness could also be due to “poor precision in estimates due to limited number of weeks of observation and few infections among participants.”
The second study found about 25% of 43,000 COVID-19 cases reported in Los Angeles between May and July were breakthrough cases, or cases of COVID-19 in someone who is fully vaccinated.
The study showed that hospitalizations were significantly lower for those who had been vaccinated, with those who had not been vaccinated 29 times more likely to be hospitalized than those who had.
In addition, those who were not vaccinated were also about five times more likely to be infected with the virus.
The studies were released a day after the U.S. Food and Drug Administration gave its full approval to the Pfizer-BioNTech COVID-19 vaccine. The CDC and the FDA recommended a third dose of the vaccine for those who are immunocompromised on Aug. 13, and for everyone who has been fully vaccinated on Aug. 18.
A booster shot is expected to be available to fully vaccinated Americans who got their second dose at least eight months prior, beginning Sept. 20.
According to the CDC, more than 171 million people in the United States are fully vaccinated against COVID-19. Around 201.7 million people have received at least one dose of either the Pfizer or Moderna vaccine.
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