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The COVID-19 Delta Variant: What to Know

Medically Reviewed by Dan Brennan, MD on January 03, 2023

The highly transmissible COVID-19 delta variant, also known as B1617.2, was found in India in October 2020. It was the dominant strain in the U.S. and several other countries during the fall of 2021, but by March of 2022 accounted for 0 cases in the U.S., having already been overtaken by the even more contagious Omicron variant.  

What to Know

It spreads quickly. By January 1, 2022, the Delta variant had reached more than 183 countries, overtaking the Alpha variant in the number of cases in many of those countries, including the U.S. The variant was thought to be 55% to 90% more transmissible than previous COVID-19 variants and more likely to result in hospitalization. Experts believe Delta was anywhere from 30% to 100% more infectious than alpha.

Researchers are still unsure why the Delta variant was so much more transmissible than others. They think changes in the variant’s protein might make it easier to enter human cells. Another early study suggests that a mutation in the Delta variant might help it blend better with human cells once it attaches itself. If it can easily blend with your cells, it’s able to infect more of them and overpower your immune system.

It seems to have affected younger people more often. In the United Kingdom, studies showed that children and adults under 50 were 2.5 times more likely to become infected during Delta’s dominance of infection.

Symptoms appear to be more severe and happen faster. People may be more likely to end up in the hospital if they have the Delta variant. Studies suggest it might have almost double the risk of hospitalization than the Alpha variant.

In China, doctors say patients with the Delta variant are sicker than those they treated early in the pandemic. And their condition seems to go downhill much faster.

The top symptoms reported on include:

A cough became less common and loss of smell no longer listed in the top 10 common symptoms anymore with the Delta variant. Researchers had concerns that people would mistake symptoms for a bad cold and avoid quarantine, helping the variant spread.

How to Protect Yourself

Getting vaccinated was effective against the delta variant. Reports suggest two doses of the Pfizer-BioNTech vaccine provided 79% protection against delta variant infection. It was up to 96% effective against hospitalization if infected.

Show Sources

World Health Organization: Tracking SARS-CoV-2 variants

GISAID: hCoV-19 Tracking of Variants

Gavi Vaccine Alliance: "Five things we know about the Delta variant (and two things we don't)"

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