What Are Covid-19 Variants?

Update: As of November 2021, the World Health Organization (WHO) has identified a new variant named Omicron as a “variant of concern.” According to WHO, the variant has a high number of mutations compared to other strains. Scientists are working to determine if Omicron causes more severe illness, if it is more contagious than other strains, and if current vaccines are effective against this new form of the virus.

In April 2020, nearly half of the 1,000 beds at Yale New Haven Hospital in Connecticut were occupied by people battling Covid-19. A year later, in April 2021, that number fell to four hospitalized patients, according to Dr. Perry Wilson, a physician who works at the hospital. But then the count began to climb. By August 17, 2021, the number of Covid patients was up to 61. What was going on?

It turned out that in almost all the August cases, patients were infected with a new strain, or form, of the virus, called the Delta variant. The new strain is a much more contagious form of Covid-19, meaning it spreads more easily from person to person. As of early October 2021, the Delta variant was the most common version of the virus circulating in the United States—accounting for more than 90 percent of Covid-19 cases. Here’s how this and other variants can arise.  

Like all viruses, the one that causes Covid-19 is a tiny, non-living particle that invades and reproduces inside a living cell. In the cell, the virus makes thousands of identical copies of itself. This replication process is a lot like photocopying a piece of paper over and over, says Wilson. Eventually, one of the photocopies won’t be perfect—the ink might smudge, or the paper might be off-center. Similarly, no virus is copied perfectly every time. Sometimes during copying, mistakes arise in the virus’s genetic code—the instructions for how a virus or other organism functions. These changes are called mutations. 

Most mutations are minor and don't significantly affect how a virus functions, says Allison Greaney, a researcher at the University of Washington who studies the evolution of viruses. A mutation might even weaken the virus. But sometimes, a virus can acquire mutations that make it more infectious. That’s what happened with the Delta variant.

Since the start of the pandemic, different variants of Covid-19 have emerged. The Delta strain was first detected in 2020. Now it has spread to 185 countries. As of October 2021, this variant has become the dominant version of the coronavirus that causes Covid-19 infecting people around the world. 

Scientists determine a variant’s infectiousness by calculating its reproductive number, says Greaney. That’s the number of people, on average, who will catch the virus from one infected person (see Spreading Infection, below). The original strain of Covid-19 has a reproductive number of 1.5 to 3, meaning one infected person will likely pass the disease on to 1.5 to 3 people. In contrast, Delta has a reproductive number of 5 to 8. 

“Whenever a variant is especially effective at infecting people, we say that it outcompetes other versions of the virus,” says Greaney. Variants with lower reproductive numbers cannot compete with Delta, she says. All over the world, these variants are disappearing as Delta outcompetes them. 

Currently, there are three vaccines approved in the U.S. to protect against Covid-19 infections. The Pfizer vaccine is available to those 12 and up, while the Moderna and Johnson & Johnson vaccines are currently available for those 18 and up. But as the virus changes, these vaccines can become less effective in preventing infection. Doctors have noticed that the Delta variant appears to have increased the risk of breakthrough infections—cases where fully vaccinated people become infected with the virus that causes Covid-19. Fortunately, the symptoms from these infections are usually mild. Unfortunately, this means that even vaccinated people can potentially become infectious and spread the disease. But according to the U.S. Centers for Disease Control and Prevention (CDC), vaccines are still an important defense against serious illness. Right now, most Covid-19 cases that result in hospitalizations and deaths in the U.S. are among people who are not vaccinated.  

Someday, a variant of the virus may emerge that’s completely resistant to current vaccines.  That’s a key reason that the CDC encourages people to continue to wear masks and practice social distancing. If the virus is free to move from person to person, this gives it even more opportunities to multiply—and potentially mutate into something more dangerous.