Vaccines cut chance of being infected with delta variant by half, UK study finds

A healthcare worker vaccinating a person.
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People who are fully vaccinated with a two-dose coronavirus vaccine have a 50% to 60% reduced risk of being infected with the delta variant, even asymptomatically, compared with unvaccinated people, according to a new study conducted in England. 

The study examined nearly 100,000 people who took COVID-19 swab tests at home between June 24 and July 12. In that sample group, 527 people tested positive for the coronavirus and 254 of the samples were genetically analyzed; all of the sequenced samples turned out to be the highly transmissible delta variant.

Once the researchers adjusted for factors such as age, they found that people who received two vaccine doses were 49% as likely to test positive for the coronavirus, even without symptoms, compared with people who were unvaccinated and that vaccinated people were 59% less likely to test positive with symptoms.

Related: Delta variant: Your questions answered

The findings, which were posted as a preprint and haven't been peer-reviewed yet, are the newest results from Imperial College London's "Real-time Assessment of Community Transmission," or REACT-1, an ongoing coronavirus monitoring study. 

"These findings confirm our previous data showing that both doses of a vaccine offer good protection against getting infected," Paul Elliott, director of the REACT program from Imperial's School of Public Health, said in a statement. The researchers didn't untangle the effectiveness of specific vaccines. 

Their findings conflict with previous studies. For example, a study conducted by Public Health England found that the Pfizer-BioNTech vaccine was 88% effective against symptomatic disease caused by the delta variant (people vaccinated were 88% less likely to develop symptomatic infection compared to people who were unvaccinated), compared with about 93% effective against the alpha variant, the previous dominant variant. That study found that the two-dose AstraZeneca vaccine was 60% effective against the delta variant, compared with 66% against the alpha variant, Live Science previously reported

Meanwhile, early data from Israel suggested that the Pfizer-BioNTech vaccine was 64% effective against symptomatic disease caused by the delta variant, and data from Canada found it was 87% effective against symptomatic disease, according to an internal presentation from the Centers for Disease Control and Prevention. But newer data from Israel found that the efficacy of the Pfizer-BioNTech vaccine against Delta slipped to 39% (but is still 88% effective against hospitalization and 91% protective against severe illness), according to CNBC.

The new study also found that vaccinated people had a smaller viral load on average, meaning they likely shed less virus and are less contagious than unvaccinated people. That result differs from other data that suggested the delta variant caused similar viral loads in the unvaccinated and in vaccinated people who test positive (so-called breakthrough cases), Live Science previously reported

"The delta variant is known to be highly infectious, and as a result, we can see from our data and others' that breakthrough infections are happening in fully vaccinated people," Steven Riley, a professor of infectious disease dynamics at Imperial College London, said in the statement. "We need to better understand how infectious fully vaccinated people who become infected are, as this will help to better predict the situation in the coming months, and our findings are contributing to a more comprehensive picture of this."

The researchers also found that the trends between infections and hospitalizations, which had weakened in the spring, were converging again, according to the statement. That could be due to the dominant variant switching from alpha to delta and more younger people, who may be less likely to be vaccinated, becoming hospitalized than before.

Young people ages 13 to 24 had the highest infection rate, and people 75 and older had the lowest infection rate. Roughly 50% of the infections occurred in people ages 5 to 24, even though they make up only a quarter of the population, Riley told Reuters. 

"Today's report shows the importance of taking personal responsibility by self-isolating if you are contact traced, getting tested if you have symptoms and wearing face coverings where appropriate," U.K. Health and Social Care Secretary Sajid Javid said in the statement. "I urge anyone who has yet to receive a vaccine to get jabbed and take up both doses — the vaccines are safe, and they are working."

Originally published on Live Science.

Yasemin Saplakoglu
Staff Writer

Yasemin is a staff writer at Live Science, covering health, neuroscience and biology. Her work has appeared in Scientific American, Science and the San Jose Mercury News. She has a bachelor's degree in biomedical engineering from the University of Connecticut and a graduate certificate in science communication from the University of California, Santa Cruz.