July 2, 2021 -- People who have received a COVID-19 vaccine and later become infected with the coronavirus tend to have a lower viral load according to a new study published Wednesday in the New England Journal of Medicine.
That means they are likely to be less contagious than unvaccinated people and don’t transmit the virus as much to others. In addition, researchers found that vaccinated people had a shorter and milder course of COVID-19.
“Even when people get vaccinated and did get infected, they were less likely to have an illness that causes a fever,” Mark Thompson, one of the lead authors and an epidemiologist at the CDC, told The Wall Street Journal.
“There’s reason to think that even if you get infected, you have less virus to spread around and infect other people,” he said.
The research team followed 3,975 health care workers and first responders who underwent weekly testing between mid-December and mid-April. COVID-19 was detected in 204 participants, including five who were fully vaccinated and 11 who were partially vaccinated with either the Pfizer or the Moderna shots.
The fully or partially vaccinated participants had a 40% lower viral load than the unvaccinated participants, as well as a 66% lower risk of having detectable virus for more than one week. They also had a 58% lower risk of fever symptoms.
None of the vaccinated people who contracted COVID-19 were hospitalized, and they all had mild or moderate cases. They also had a shorter duration of illness, with about two fewer days spent sick in bed and six fewer days of symptoms.
Based on study results, researchers calculated that the vaccines were 91% effective at preventing infections among fully vaccinated people and 81% effective among partially vaccinated people.
In addition, researchers sequenced 10 of the 16 cases found in fully or partially vaccinated people. They found three variant cases, which all involved the Epsilon variant first identified in California. Study samples were collected before the contagious Delta variant, which was first identified in India, arrived in the U.S. this year, The Wall Street Journal reported.
“Vaccines not only are highly effective in preventing SARS-CoV-2 infection but also may mitigate the effects of breakthrough infections,” researchers wrote, which is “especially important to essential and frontline workers, given their potential to transmit the virus through frequent close contact with patients, coworkers, and the public.”