After months of steady decline in COVID-19 cases, the United Kingdom is experiencing an uptick due to the Delta variant. Also known as B.1.617., the coronavirus variant has taken over as the dominant strain in the UK, and health officials warn that the same thing happen in the United States. The Delta variant is proving more harmful than the original, or Alpha, version of the virus, says Jennifer Horney, PhD, MPH, founding director of the epidemiology program at the University of Delaware.

“We know a few things about the Delta variant that are concerning,” says Dr. Horney. “It’s highly transmissible and there is evidence of increased risk of hospitalization.”

The Delta variant emerged during a recent surge in India, where only 3.3 percent of the population is fully vaccinated against COVID-19, highlighting the need for increased vaccination rates globally.

Currently, the Delta variant has been reported in 60 countries and accounts for less than 6 percent of COVID-19 cases in the United States. Underscoring the need to get vaccinated, Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases (NIAID) and the chief medical advisor to President Joe Biden, says we must do everything we can to prevent the Delta variant from becoming the dominant variant in the U.S. “If you had your first dose, make sure you get that second dose,” he said during a Tuesday briefing with the White House pandemic response team. “And for those who have been not vaccinated yet, please get vaccinated.”

“Variants occur when the virus is transmitted from person to person. The virus mutates slightly, that’s normal, we expect that,” says Dr. Horney. “The more it transmits from person to person, the more opportunity it has to mutate, so we need to use the vaccines to stop that transmission.”

Listen to a biochemist explain how vaccines work:

Are the vaccines effective against the Delta variant?

Pre-print research published on May 25 on MedRxiv, found that both the Pfizer-BioNTech and AstraZeneca vaccines are effective against the Delta variant, but not as effective, “particularly when people are in the period before they are considered ‘fully’ vaccinated, less than two weeks after their second dose,” says Dr. Horney.

After the second dose, the Pfizer vaccine is 88 percent effective against symptomatic disease from the Delta variant, compared to 93 percent effective against the Alpha variant. After the second dose, the AstraZeneca vaccine is 60 percent effective against symptomatic disease from the Delta variant, compared to 66 percent effective against the Alpha. Three weeks after one dose, both vaccines were only 33 percent effective against symptomatic disease from the Delta variant, compared to about 50 percent effective against the Alpha variant, emphasizing the risks of skipping of the second dose.

There is no current data showing whether or not the Moderna and Johnson & Johnson vaccines are effective against the Delta variant. However, Dr. Fauci expressed certainty that the Moderna Delta-variant efficacy would be similar to that of Pfizer given that both are mRNA vaccines. And recent research published in Nature shows that the Johnson & Johnson vaccine is effective against the Alpha, Beta, Gamma, and Epsilon COVID-19 variants, suggesting similar results can be expected for the Delta variant.

Who is most at risk from the Delta varient?

In the UK, peak transmission of the Delta variant is happening among individuals age 12 to 20.

“Since adolescents [age] 12 to 15 just became eligible on May 12, that population needs to quickly get fully vaccinated to be protected against the Delta variant,” says Dr. Horney. “While [the Centers for Disease Control and Prevention] did say that vaccinated people could take their masks off, anytime they are in a place with kids under 12, then they’re inherent with people who are unvaccinated and so we should keep them on. We have to remain cautious when we’re outside of our household or among people who aren’t yet eligible to be vaccinated.”

Not specific to the Delta variant, the rate of COVID-19 cases in the U.S among the unvaccinated is very high right now, says Dr. Horney. And while the overall count of COVID-19 case numbers is declining, the rate of cases among the unvaccinated remains higher, according to a Washington Post analysis.

“The problem is that COVID-19 cases are still being reported using the full population in the denominator, rather than only the unvaccinated,” says Dr. Horney. “We’re actually seeing infection and hospitalization rates in many states that are similar to what they were in the January-February surge period. A virus is going to find people who are not vaccinated and continue to spread.” And as it spreads, it will continue to mutate.

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