6 Things to Know About the COVID-19 Delta Variant

COVID-19 Delta is a SARS-CoV-2 “variant of concern.” Variants of concern have a collection of random mutations that cause the virus to act differently than the original novel coronavirus and can be more contagious or resistant to treatment. The Delta variant started in India and spread quickly across the globe. The first cases in the United States were confirmed around June, and within a month it was causing all new U.S. coronavirus infections and COVID-19 illness.

(Variants of concern are denoted by the Greek alphabet—Alpha, Beta, Gamma, Delta, Epsilon, and so on—rather than the country from which they are initially discovered.)

The COVID-19 Delta variant, also designated B.1.617.2, is estimated to be 60% more contagious than the Alpha variant, which itself is 50% more contagious than the original virus that started the pandemic. Based on case rates here and in the United Kingdom, the Delta variant is 2 to 4 times more contagious than the original virus. That means one infected person can potentially infect six susceptible people (such as unvaccinated people and vaccinated, but immunocompromised people).

The exponential growth of the Delta variant caused a rise in COVID-19 hospitalizations, which is expected based on the shear number of infections. But does the Delta variant cause more severe COVID-19 than earlier versions of the virus? There's no clear answer to that question.

In a Scottish study of about 20,000 COVID infections (mainly unvaccinated individuals), the Delta variant doubled the risk of hospitalization compared to the Alpha variant. However, the overall trend in U.S. hospitalizations is not significantly between the Delta variant and earlier versions.

The Centers for Disease Control and Prevention (CDC) reports the Delta variant "might cause more severe illness than previous strains in unvaccinated people."

The difference in protection among unvaccinated, partially and fully vaccinated people is much larger with the Delta variant than Alpha variant. In other words, even a partially vaccinated individual is more vulnerable to the Delta variant than to the Alpha and other variants of concern. Likewise, people who recovered from COVID-19, but have not been vaccinated against it, have a higher risk of reinfection with Delta compared to Alpha and older versions of the virus (if the first infection occurred more than six months earlier).

Being fully vaccinated offers the most protection, but vaccinated people can still get infected and sick from the Delta variant because vaccination is not 100% effective. Vaccination helps prevent COVID-19 Delta hospitalization or death, which appears to be 10 times more likely for unvaccinated than fully vaccinated individuals, according to several studies summarized by the CDC.

A booster shot of an mRNA-type COVID-19 vaccine strengthens immunity to the Delta variant. The U.S. Food and Drug Administration (FDA) also authorized an additional COVID-19 vaccine dose for immunocompromised people.

When the Delta variant took over England, there was a steep rise of cases in children. This was due, in part, to the fact that children younger than 12 are not yet able to receive the COVID-19 vaccine. The number of U.K. cases caused by the Delta variant was higher in younger (ages 5 to 49) vs. older (ages 50 or older) age groups, but cases rose at a similar rate in both groups.

Most children develop mild (or no) symptoms, but some children require hospitalization, even intensive care. The Delta variant does not appear to cause more severe disease in children, compared to the Alpha variant or the original virus. Rather, the rise in U.K. and U.S. hospitalizations may be because there are simply more Delta infections occurring in the large pool of unvaccinated children.

The most common symptom in people with a confirmed COVID-19 infection is headache—according to data collected from a COVID-19 symptom study app since May 2021, when Delta was the most common circulating variant in the United Kingdom. COVID-19 Delta symptoms, including sore throat, runny nose, and fever may be easily mistaken for those of flu or the common cold (however, it is rare to develop a fever with a cold). COVID-19 testing is the only way to know for sure that you have COVID-19.

Laboratory-made antibodies that target SARS-CoV-2 have emergency use authorization under certain circumstances for preventing severe COVID-19. Additional laboratory studies show that these targeted monoclonal antibody treatments, particularly bamlanivimab, are less efficient at blocking the Delta version of SARS-CoV-2’s spike protein compared to the spike protein from the original virus.