Researchers at the Imperial College of London studied 621 people who had mild symptoms and found that the peak viral load of vaccinated individuals is similar to that of the unvaccinated. The study further found that 25 percent of vaccinated household contacts contracted COVID-19, while 38 percent of unvaccinated individuals were diagnosed with the disease.
The delta variant is responsible for 99 percent of COVID-19 cases in the United States. It is also the most common cause of COVID-19 around the world.
Ajit Lalvani, a professor of infectious diseases at Imperial College London and a co-leader of the study, said that vaccination alone is not enough to prevent people from being infected with the disease and spreading it in household settings.
"Our findings suggest that vaccination is not sufficient to prevent transmission of the delta variant in household settings with prolonged exposures," the researchers wrote.
Vaccinated people are not safe from the delta variant
Epidemiologist and professor of data science at the University of Edinburgh, Rowland Kao, said that the results of the study are consistent with what people already know about the delta variant: It can infect even those who have been vaccinated. (Related: Vast majority of delta variant deaths occurring in the "fully vaccinated.")
The same findings bolstered the case that many scientists are making: Natural immunity from infection is better than vaccine-induced immunity. The study showed that immunity from full vaccination wanes in as little as three months. This is why booster shots for older and more vulnerable populations are being offered six months after their second shot.
The researchers wrote: "Increasing population immunity via booster programs and vaccination of teenagers will help to increase the currently limited effect of vaccination on transmission, but our analysis suggests that direct protection of individuals at risk of severe outcomes, via vaccination and non-pharmacological interventions, will remain central to containing the burden of disease caused by the delta variant."
In the U.S., the Centers for Disease Control and Prevention (CDC) quietly updated its vaccine guidance for immunocompromised individuals, saying that they can receive booster doses at least six months after completing their primary vaccination series.
Experts say that extra doses or repeated infections contribute to longer immunological memory. However, more data is needed to confirm this claim.
The authors of the study did not analyze the infections based on the type of vaccines people received. According Maria Zambon, the head of influenza and respiratory virology in the U.K.'s Health Security Agency, there are still over 300 vaccines that are currently in development. They're hopeful that future versions may be better at preventing the transmission of COVID-19.
Earlier this month, former Food and Drug Administration (FDA) Commissioner Scott Gottlieb predicted that the "pandemic phase" of COVID-19 is about to end and the world will soon transition to an "endemic" phase as people learn to live with the virus.
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