What does the Delta variant mean for the fully vaccinated?

The U.S. is seeing a surge in COVID-19 cases driven by the Delta variant. Just how dangerous is it? Here's everything you need to know.

What is the Delta variant?

The Delta variant of SARS-CoV-2 — the coronavirus that causes COVID-19 — was first identified in India in December 2020. After ravaging India and sweeping through Great Britain, the Delta variant is now the dominant strain in the U.S., accounting for more than 83 percent of COVID-19 infections.

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Is Delta more dangerous?

The World Health Organization calls Delta "the fastest and fittest" of the variants, and they don't mean it as a compliment. The Delta mutation modified the protein spikes the coronavirus uses to attach to and infect cells, and those mutations make it at least twice as transmissible as the original strain. Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, called the Delta variant "one of the most infectious respiratory viruses we know of and that I have seen in my 20-year career." The Delta strain doesn't appear to be intrinsically more fatal, but people infected with the variant have a viral load 1,000 times greater than those infected with earlier mutations, according to preliminary research, and they shed more virus for a longer period of time. The massive quantity of virus means more chances to infect a person's cells, and the more the Delta variant spreads in a community, the higher the odds an individual will come in contact with infected people and get blasted with an amount of coronavirus that will make them sick.

Do the vaccines still work?

Thankfully, yes. All three vaccines approved for emergency use in the U.S. — Pfizer/BioNTech, Moderna, and Johnson & Johnson — and others not yet approved — have proved extremely effective against the Delta variant. The vaccines appear to be about 90 percent effective at preventing serious illness and hospitalization, and 99.5 percent of U.S. COVID-19 deaths this year have been among unvaccinated people. But the vaccine isn't 100 percent effective and it isn't an impenetrable shield against COVID-19, especially when the virus is spreading freely.

Should vaccinated people worry about "breakthrough" infections?

The CDC no longer tracks mild or moderate "breakthrough" infections — instances in which vaccinated individuals contract the virus — so we don't know exactly how common they are. But the CDC does track severe infections among fully vaccinated Americans, and as of July 12, there were 5,492 cases reported, including 1,063 deaths. That is a miniscule fraction of the 159 million vaccinated Americans, but breakthrough infections do happen — and are expected to happen. At the same time, "they tend to be mild — they tend to be more like a cold," says Dr. Carlos del Rio, infectious disease epidemiologist at Emory University.

These charts illustrate the difference in fatality rates between waves of COVID-19 infections before and after a country is significantly vaccinated.

Honestly, "I think we are misusing the term breakthrough," says Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia. "If someone who is fully vaccinated is subsequently hospitalized or killed by the virus, that's a breakthrough case," but in the overwhelming majority of cases, "the vaccine is still doing what it is designed to do — keep people out of the hospital and out of the morgue."

Should vaccinated people wear masks?

The concensus among most public health experts is that wearing a mask can't hurt, especially in situations where the risk of transmission is higher, like crowded enclosed spaces, or if you're immune compromised. "Avoiding crowded spaces and wearing a mask when you're indoors and don't know the vaccination status of those around you is a good idea," writes Tara Parker-Pope at The New York Times. On the flip side, the CDC, citing limited evidence, says vaccinated people are less likely to infect others than unvaccinated people with asymptomatic cases. That would make sense, because people who got vaccinated generally have a lower viral load, which "makes it less likely for you to transmit — not impossible by any means but less likely," says Dr. Monica Gandhi at University of California, San Francisco.

How about booster shots?

The Food and Drug Administration has not authorized a booster shot of a COVID-19 vaccine, and health experts say it won't be necessary to counter the Delta variant or any other probable mutations in the near future. But on Friday, the Biden administration announced it had purchased an additional 200 million Pfizer doses just in case the data proves otherwise and boosters become necessary in the fall or spring. A CDC advisory panel has expressed preliminary support for giving booster shots to some immunocompromised Americans. If a future variant thwarts the current vaccines, a new booster vaccination may be needed.

Is that likely to happen?

It's hard to say, but experts are worried. The Delta variant is also suring in Asia, the Middle East, Africa, Latin America, and Europe, and "the more uncontrolled spread, the more risk of even more dangerous variants," says Dr. Tom Frieden, former CDC director. "Variants remain the wild card for the trajectory of the pandemic. Delta may not be the worst strain the virus deals us." Public health experts will be watching hospitalization data carefully in coming months for signs of a surge in vaccinated people becoming seriously ill, which might indicate a change in the virus. "Right now, it's Delta," says MIT medical director Dr. Cecilia Stuopis. "In the fall it may be Kappa or some other, fitter, variant. We can't be complacent. We are going to be living with this virus in some form or another for the foreseeable future, but our actions right now can determine what that future will be."