6 Things to Know About the COVID-19 Delta Variant

COVID-19 Delta is the latest “variant of concern” of SARS-CoV-2, the virus that causes COVID-19. 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. There are several reasons why the COVID-19 Delta variant—responsible for the COVID crisis in India—has public health experts and doctors concerned, including its transmissibility and disease severity.

(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 Delta variant, also designated B.1.617.2, originated in India, rapidly spread across the globe, and quickly became the dominant strain in the U.K. and several other countries. The Delta variant is responsible for more than 83% of new U.S. COVID-19 infections.

The COVID-19 Delta variant 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. Estimates are that Delta is 2 to 4 times more contagious than the original virus. A CDC report places the Delta variant as or nearly as contagious as chickenpox, where a single infected person can infect up to 10 to 12 people susceptible to the virus, such as unvaccinated or vaccinated but immunocompromised individuals. With measles, a single infected person can infect 12 to 16 susceptible people.

Due to increased transmissibility, health officials are asking vaccinated people to once again don masks in indoor, public spaces to reduce the spread of the virus.

The exponential growth of the Delta variant is causing a rise in COVID-19 hospitalizations, which is expected based on the shear number of infections. But there is some evidence the variant virus causes more severe disease. 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, more cases studies are necessary to conclude that the Delta variant causes more severe disease than the Alpha variant. The CDC reports the Delta variant is "likely more severe" than the prior circulating strains.

The changes in the Delta variant allow it to evade the immune system to some degree. 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 received the vaccine, 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.

It's true that fully vaccinated people are less protected from infection caused by the Delta variant than the form of the virus the vaccines were designed to guard against; however, vaccination is still up to 90% effective at preventing hospitalization or death.

Most of the studies behind the numbers are with the Pfizer-BioNTech vaccine, but Moderna expects their version of the COVID-19 vaccine to be similarly effective, based on laboratory studies not yet peer-reviewed in a scientific journal. The one-shot Johnson & Johnson (J&J) COVID-19 vaccine is about 60% effective against the Delta variant (compared to 66% efficacy, overall, against other common circulating strains). In another small study, antibody responses to the Delta variant in people who received the J&J vaccine persisted at least eight months, according to a company press release. Being fully vaccinated offers the most protection against COVID variants.

In England, the rise of COVID-19 Delta cases in children is due, in part, to the fact that children younger than 12 are not yet able to receive the COVID-19 vaccine, at least under its emergency use authorization. The number of cases caused by the Delta variant is higher in younger (ages 5 to 49) vs. older (ages 50 or older) age groups, but cases are rising at a similar rate in both groups. With vaccination age restrictions, combined with the fact that the Delta variant is more transmissible, a rise in COVID-19 cases in children, which can result in hospitalization, is expected in the U.S. as well.

The most common symptom in people with a confirmed COVID-19 infection is headache—according to data collected from a U.K. COVID-19 symptom study app since May when Delta was the most common circulating variant in the U.K. COVID-19 Delta symptoms may be easily mistaken for those of the common cold, including sore throat, runny nose, and fever (however, it is rare to develop a fever with a cold). These COVID-19 Delta symptoms are different from cough and loss of smell symptoms caused by other variants and original SARS-CoV-2.

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.