Why are boosted Americans testing positive for COVID-19 more often?
Since late February, Americans who have gotten a booster shot appear to be testing positive for COVID-19 more often than those vaccinated without the extra shot, according to Centers for Disease Control and Prevention data.
This is based on numbers up until the week of April 23, which is the most recently released CDC data comparing case rates of those boosted, vaccinated and unvaccinated against the coronavirus. Ultimately, the numbers, which are updated monthly, showed those unvaccinated had the highest case rates overall.
Meanwhile, about 119 out of 100,000 boosted individuals tested positive for COVID-19 during the week of April 23, according to CDC data. In comparison, 56 out of 100,000 individuals vaccinated with only a primary series tested positive.
But why are case rates higher for boosted individuals than for those vaccinated without a booster?
Dr. Sheela Shenoi, an infectious disease doctor and assistant professor at Yale School of Medicine, told McClatchy News over the phone that “there’s no biological reason that people who have had [the vaccine] and boosters are going to be at increased risk for COVID.”
“These numbers are not telling us the whole truth,” Shenoi said.
The CDC wrote in a summary accompanying its data that “several factors likely affect crude case rates” and this makes “interpretation of recent trends difficult.”
Here are some potential factors to keep in mind, according to health experts, when looking at the data.
At-home testing:
“The wide availability of at-home tests has substantially muddied the waters, because these do not necessarily show up in official figures,” Bill Hanage, an associate professor of epidemiology at Harvard University’s T.H. Chan School of Public Health and a co-director for the Center for Communicable Disease Dynamics, told McClatchy News in a statement.
“Individuals receiving boosters may be more likely to have their cases counted,” Hanage said.
Hanage said this is because “just in being boosted, they are displaying ‘health seeking’ behavior” and “they are more likely to have contact with healthcare and get a test that ends up in official stats.”
Behaviors: Shenoi said it’s possible that individual behaviors might influence why CDC data shows those with booster shots are testing positive more than those vaccinated with a primary vaccine series.
Those boosted may feel more comfortable and safe, according to Shenoi, and as a result, they might be taking less COVID-19 precautions such as masking and social distancing “because they feel like they’re protected by the booster.”
Prior infections and those at higher risk:
It’s possible that people who have gotten their primary vaccine series but not a booster “are more likely to have been recently infected during the first omicron wave,” Hanage said. With that “additional immunity from that infection,” they are less likely to be infected now, he added.