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The Delta Variant Now Makes Up More Than 50% Of COVID-19 Cases In The US

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The Centers for Disease Control and Prevention officials reported Tuesday that the delta variant made up more than 50% of COVID-19 cases in the U.S. between June 20 and July 3 — up 30.4 from the previous two-week period.

What is the latest news and research about variants of SARS-CoV-2? How worried should we be? Who is most at risk?

Are the vaccines in circulation effective against Delta and other variants? Will booster shots be necessary?

Why do viruses like SARS-CoV-2 mutate? Are variants like delta and lambdamore transmissible or more likely to cause severe illness?

Could variants affect a return to pre-pandemic “normalcy"? Should people take additional precautions at work or school, when socializing or eating indoors? Does the spread of variants make it less safe to travel?

What is the latest guidance from the CDC and the World Health Organization, and why do they differ when it comes to mask policy?

What is the White House's position and strategy for addressing these potential threats? Is getting all Americans vaccinated enough to curb the impact of COVID variants in the U.S., or will inoculation of the global population be necessary to stop their spread?

This Q&A has been edited for length and clarity. It includes answers from all three guests.

What is a variant?

When viruses like SARS-CoV-2 circulate in any host, their genetic information changes. It's a normal part of evolution for an organism to make mistakes, and viruses are really good at it. So any time a virus infects you, it changes. In the early parts of this outbreak, the changes were minimal. Now, they're getting to be more in number. When we say variant, all we mean is that if I took a virus from you and from me, and they were different, genetically one would be a variant of the other. That's all it means.

What does it mean that the delta variant is “more sticky” than the dominant form of COVID-19?

That's a hypothesis, that the virus is better able to stick to our cells, and that explains its spread. We've had a series of variants — alpha, beta, gamma, delta and lambda is next. I think everyone is familiar with this idea of survival of the fittest, an organism will do better than another and the variants displace the ones before because they're more fit. I think the reason for that is not because of stickiness, but it has to do with evasion of immune responses, antibody responses by the variant. It's the same thing that happens every few years with influenza viruses. New variants arise, they evade our immune responses, and they displace the previous viruses. So they're more fit; they're not really more transmissible.

Will the vaccine protect against variants?

If you want to talk about protection, you need to be very specific. You can't just say X percent effective. You have to say effective against what? Effective against infection, effective against mild moderate severe disease or death. Most human vaccines do not prevent infection. They prevent disease. The flu vaccine, for example, prevents severe disease, but it doesn't prevent the infection. So reports that vaccines COVID vaccines are currently allowing infection is not really an issue. Most of the vaccines as far as I can tell, prevent moderate-to-severe disease and death, and that's exactly what we want no matter what the variant, moderate to severe disease and death is prevented by the vaccines, and that's a good reason for everyone to get them.

Why does COVID-19 impact people differently?

When this virus first entered humans, everybody was vulnerable to it. There was no immunity in the population at all, as we have for other human viruses that are circulating. And a particular older folks are particularly vulnerable. You don't have a great immune system as you age. Your immune system gets weaker and weaker. And so they initially showed the most severe disease with less severe disease as you get younger. Now we understand that even the very young can get very severe diseases. Kids are at less risk for getting a severe disease than adults, but they can develop severe inflammatory disease, and so it's a good reason to keep them safe. The reasons for this, though, are really unknown. As you know, the most severe disease comes from what we call an over exuberant immune response to infection. Many people can control the infection within two weeks. They have very mild or asymptomatic disease, others overreact.


  • Dr. Luis Ostrosky, M.D., professor and chief of infectious diseases at McGovern Medical School at UTHealth in Houston
  • Vincent Racaniello, Ph.D., virologist and professor in the Department of Microbiology and Immunology at Columbia University
  • Tanya Lewis, senior editor covering health and medicine for Scientific American

"The Source" is a live call-in program airing Mondays through Thursdays from 12-1 p.m. Leave a message before the program at (210) 615-8982. During the live show, call 833-877-8255, email thesource@tpr.org or tweet @TPRSource.

*This interview was recorded on Wednesday, July 7.

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