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The highly transmissible coronavirus variant called delta is now the dominant strain in the United States, and continues to gain steam.

Modeling shows the variant now accounts for 83.2 percent of all new infections in this country, according to the most recent data from the Centers for Disease Control and Prevention.

CDC Director Rochelle Walensky said during a White House news briefing Thursday that delta is “more aggressive and much more transmissible” than previous strains and is “one of the most infectious respiratory viruses we know of and that I have seen in my 20-year career.”

“We are yet at another pivotal moment in this pandemic, with cases rising again and some hospitals reaching their capacity in some areas. We need to come together as one nation, unified in our resolve to protect the health of ourselves, our children, our community, our country and our future with the tools we have available,” she said.

The good news, Walensky has said, is that all three coronavirus vaccines authorized in the United States offer strong protection against severe disease and death from covid-19. Preliminary data from several states over the past several months suggests that 99.5 percent of covid-19-related deaths occurred among unvaccinated people, she said.

Here are answers to commonly asked questions about the delta variant and how to protect yourself.

Frequently Asked Questions

  • What is the delta variant?
  • Why is the delta variant a concern?
  • How effective are the vaccines against the variant?
  • How will the delta variant affect the United States?
  • Do symptoms from delta infections differ from infections from other variants of the coronavirus?
  • Will booster shots be needed?

What is the delta variant?

The delta variant, also known as B.1.617, was first detected last year in India, where it has been ravaging the nation and has since spread to dozens of other countries, upending plans for a return to normalcy.

Delta has several lineages with slightly different sets of mutations. One of those — B.1.617.2 — is also now the dominant coronavirus variant in the United Kingdom, where it accounts for the vast majority of all covid-19 cases in that nation.

Health experts describe delta as the most “fit” variant of the coronavirus. That means it’s likely to outcompete other variants and make more people sick from covid-19, the disease caused by the coronavirus, said Monica Gandhi, an infectious-disease expert at the University of California at San Francisco. “It’s the one that is most likely to latch onto cells in a host, and it attacks that host better than the other variants, because it can replicate itself better.”

The delta variant has become the dominant strain of coronavirus in the United States resulting in a rise in infections and hospitalizations. (John Farrell/The Washington Post)

Why is the delta variant a concern?

Early research suggests the delta variant is about 50 percent more contagious than the alpha variant, which was first identified in the United Kingdom and became the predominant variant in the United States during the spring. Alpha was already about 50 percent more transmissible than the original variant of the coronavirus first identified in Wuhan, China, in late 2019.

Although there is compelling evidence that delta is more transmissible, there is limited data on whether it is more likely to result in severe illness.

Public Health England (PHE) found that the variant may be associated with a higher risk of hospitalization, and some early research from Scotland suggests that the risk of hospitalization with covid-19 from the delta variant is about twice the risk from the alpha variant — with unvaccinated people at the greatest risk.

But health experts caution that there is not yet conclusive evidence that delta causes more severe disease. Hospitals serving areas where it is surging have reported admitting more young and middle-aged covid-19 patients, but that may be because they are less likely to be vaccinated.

How effective are the vaccines against the variant?

Real-world data suggests that all three vaccines authorized for emergency use by the Food and Drug Administration offer strong protection against severe disease and death from the delta variant, although they appear to offer less robust protection against minor to moderate infections.

Such data has come not only from the United States but also from other countries.

“I think this is a really important point, because that’s our primary goal with our vaccination effort: to prevent severe disease, to prevent hospitalization, to prevent deaths,” said William Moss, a professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

In the case of the two-dose messenger RNA vaccines, both shots are needed to mount a strong response against the delta variant. A study published Wednesday in the New England Journal of Medicine found that the Pfizer-BioNTech vaccine was 36 percent effective against symptomatic disease caused by the delta variant after the first shot of the two-shot regimen, but 88 percent effective after the second shot. This confirms earlier findings from PHE.

In terms of preventing hospitalizations from delta, a separate PHE paper suggested that the Pfizer vaccine was 94 percent effective after the first shot and 96 percent effective after the second.

Health experts said that, because the Moderna vaccine uses the same technology as the one from Pfizer, they extrapolate that it offers similar protection.

As for the Johnson & Johnson vaccine, one journal preprint showed that it had a diminished immune response to the delta variant in laboratory tests. However, other studies including one recently published in the New England Journal of Medicine suggest that it can provide robust protection for months after vaccination. Some researchers believe the Johnson & Johnson vaccine may end up showing similar results to the AstraZeneca vaccine, which was included in the PHE studies and found to be 60 percent effective against symptomatic disease and 92 percent effective against hospitalization after both doses. The AstraZeneca vaccine has not been approved for use in the United States.

“The good news is that our vaccinations are highly effective,” President Biden has said. “Fully vaccinated Americans have a high degree of protection, including against this delta variant.”

The president said the rapid spread of a more easily transmissible and potentially more dangerous variant “should cause everybody to think twice.”

“And it should cause reconsideration, especially in young people who may have thought that they didn’t have to be vaccinated, didn’t have to worry about it, didn’t have to do anything about it up to now,” he added.

How will the delta variant affect the United States?

With more than 160 million Americans now fully vaccinated against the coronavirus, the delta variant is not expected to cause massive outbreaks across the United States. Instead, health experts anticipate that it will cause surges in communities where vaccination rates are lower, posing the most serious risk to those who are older, sicker and unvaccinated.

“We’re going to have to face it throughout the United States,” said Eric Topol, a professor of molecular medicine and executive vice president at Scripps Research.

Topol said the states that are most vulnerable are the ones that have had fewer coronavirus cases and lower vaccination rates, meaning they do not have as much natural and vaccine-induced immunity. States with higher vaccination rates, such as Connecticut, Massachusetts and Vermont, have “a delta wall,” he said.

Anthony S. Fauci, the nation’s leading infectious-disease expert, has said he is “very concerned” about the variant, because it could lead to “two Americas”: one largely protected because of high vaccination rates, the other experiencing surges because of low vaccination rates.

“When you have such a low level of vaccination superimposed upon a variant that has a high degree of efficiency of spread, what you are going to see among undervaccinated regions — be that states, cities or counties — you’re going to see these individual types of blips. It’s almost like it’s going to be two Americas,” he said.

Do symptoms from delta infections differ from infections from other variants of the coronavirus?

There is little research on this, but some people have reported symptoms such as a headache, sore throat and runny nose, without the hallmark covid-19 signs, such as a loss of taste and smell.

Will booster shots be needed?

That is an open question. Federal health officials are not recommending them at this time but at least one of the vaccine makers said it plans to seek approval for a booster shot.

Data recently released earlier this month by Israel’s Health Ministry showed waning vaccine efficacy against the coronavirus in that highly vaccinated country as the delta variant became more widespread. The data showed that vaccinated people retained protection against severe disease and hospitalization, but had markedly lower protection against infection and symptomatic illness.

Citing that data, Pfizer and its partner BioNTech released a statement July 8 saying they would seek federal regulatory approval for a booster shot following studies that found such a shot caused disease-blocking antibodies to increase five to 10 times higher than after the two-shot regimen. “We continue to believe that it is likely, based on the totality of the data we have to date, that a third dose may be needed within six to 12 months after full vaccination,” the statement said.

But hours later, the Department of Health and Human Services issued a rebuke, saying “Americans who have been fully vaccinated do not need a booster shot at this time.”

The HHS statement said “a science-based, rigorous process” headed by the CDC, FDA and the National Institutes of Health would determine when or whether boosters were necessary.

What else can people do to protect themselves?

Health experts say that people who are not yet vaccinated should continue to wear masks and make plans to get vaccinated as soon as they can.

While the CDC no longer recommends masking for those who are fully vaccinated, because the vaccines provide a high degree of protection, Fauci said individuals may make their own assessments of risk based on their age, health status and circumstances.

Those living in areas where vaccination rates are low and delta cases are high may want to continue covering their faces, particularly in high-risk settings, such as indoor gatherings, areas with large crowds or places such as senior living facilities, where the consequences of transmission could be grave. Los Angeles County announced July 15 that it was reinstituted an indoor mask mandate for everyone.

The World Health Organization is still urging people around the world to mask up. Mariângela Simão, the WHO’s assistant director general for access to medicines and health products, recently told reporters: “People cannot feel safe just because they had the two doses. They still need to protect themselves.”

Gandhi said that recommendation makes sense coming from the WHO, which is dealing with very mixed populations across the globe, many with high amounts of circulating virus, low vaccination rates and less effective vaccines. Only about 13 percent of the world’s population has been fully vaccinated against the virus.

“The likelihood of getting a breakthrough infection with any variant is not just dependent on your vaccination status but the amount of virus you’re seeing circulating in your community,” she said. “It’s why health-care workers in India, even though they were fully vaccinated, still had breakthrough infections, because they were seeing so much virus.”

Health experts also urge vaccinated people to get tested for the coronavirus if they experience any symptoms related to the illness, to rule out potential breakthrough infections and to seek treatment if they test positive.

What about people who are immunocompromised?

Vaccination is still the top recommendation to protect against the virus, including the delta variant, although not all immunocompromised people may mount a robust response. But others will build immunity from the shots, and there appears to be no harm from getting immunized, said William Schaffner, medical director of the National Foundation for Infectious Diseases.

Beyond that, individuals with compromised immune responses should practice social distancing, wear masks and avoid crowds, particularly indoors. “Those things continue to pertain to these highly susceptible people,” he said.

Schaffner, a professor of infectious diseases and preventive medicine at Vanderbilt University, said it’s also important for immunocompromised people to get tested if they experience symptoms, especially as the new influenza season rolls around, so they can get treated appropriately — either for flu or covid-19. In some cases, for example, antiviral medications may be used to treat influenza and monoclonal antibodies may be used to treat covid-19, which can reduce the risk of hospitalization and death when used early in the course of the disease.

What about the risk to children, and how can parents protect them?

There is no indication the delta variant is more virulent if it infects children, said Paul A. Offit, a pediatrician and the director of the Vaccine Education Center at Children’s Hospital of Philadelphia.

However, because delta is more contagious than earlier variants, children are at somewhat greater risk of infection. Those who are 12 and older are eligible for vaccines, and federal health officials recommend they be immunized for greater protection.

Offit suggests parents make sure that unvaccinated children ages 2 or older, if not eligible for the vaccines, wear masks if they are in public spaces indoors, in accord with CDC guidance.

The CDC also recommends that parents make sure their unvaccinated children wash their hands often with soap and water, avoid close contact with those who are coughing, sneezing or complaining of feeling unwell, and limit interactions with people who are at high risk of developing severe disease.

The bottom line is that people need to take measures to protect themselves and their loved ones against the delta variant.

“If you haven’t had covid, you’re not vaccinated and you’re not wearing a mask, you’re basically asking for delta trouble. It’ll find you. It’s the most efficient form of the virus for finding hosts, by far. If you’re not vaccinated, a mask is important right now,” Topol said.

Ben Guarino contributed to this report.