What is the delta variant?

The delta variant, also known as B. 1.617.2 , was first detected in 2020 in India but soon spread worldwide, including to the United States. The variant has many sublineages with slightly different sets of mutations. But so far, none of the sublineages has spread with sufficient speed to persuade infectious-disease experts that it is clearly more transmissible than the parent delta strain. One of those sublineages, referred to scientifically as AY. 4.2 and often lumped into a group of sublineages known as “delta plus,” has also been seen in the United States. But scientists do not know whether it is truly more transmissible. Some researchers say there is currently no evidence it behaves any differently than its parent variant. Health experts once described delta as the most “fit” variant of the coronavirus. As a virus spreads, most mutations actually hinder the virus’s ability to replicate, and many others have no effect at all. But a very small number of mutations can change the structure or function of the virus in a subtle way that gives that version of the virus a competitive advantage, typically by enhancing transmissibility. Omicron has proven to be significantly more transmissible.

Why is the delta variant a concern?

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 last spring. Alpha was already about 50 percent more transmissible than the original strain of the coronavirus first identified in Wuhan, China, in late 2019, setting off a global pandemic. The delta variant changed the fight against the coronavirus as data has proven it is highly contagious, ravaging unvaccinated communities and infecting some people who are fully vaccinated when it was prominent. A study published in October in the peer-reviewed journal the Lancet Infectious Diseases found that people who were vaccinated but developed mild breakthrough infections recovered from delta more quickly than those who were unvaccinated — but could still spread the virus to other members of the household. Experts have been split on whether the delta variant causes more severe disease. That said, UK Health Security Agency (formerly Public Health England) found that delta may have been associated with a higher risk of hospitalization, and some research from Scotland suggested that the risk of hospitalization with covid-19 from the delta variant was about twice the risk from the alpha variant — with unvaccinated individuals at the greatest risk.

How effective are the vaccines against the variant?

Real-world data suggests that the Pfizer-BioNTech vaccine (which has been granted full approval by the Food and Drug Administration for people 16 and older) and the Moderna and Johnson & Johnson vaccines (which are authorized for emergency use) offer strong protection against severe disease and death from the delta variant, though they appear to offer less protection against minor to moderate infections. “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. Such data has come not only from the United States but also from other countries. In the case of the two-dose messenger RNA vaccines — Pfizer-BioNTech and Moderna — both shots are needed to mount a strong response against the delta variant. That said, some studies have shown a drop in vaccine effectiveness against delta infections. The most recent research from UK Health Security Agency found that two doses of the vaccines are 40 to 60 percent effective at preventing symptomatic disease from the delta variant six months after immunization and 90 to 95 percent effective after a third dose. In addition, two doses of the vaccines are up to 85 percent effective against hospitalization and up to 99 percent effective against death, with three doses providing even stronger protection, the data shows. As for the Johnson & Johnson vaccine, one journal preprint showed that it had a diminished immune response to the delta variant in laboratory tests. Other studies, including one published in the New England Journal of Medicine , suggest that it can provide robust protection for months after vaccination. All of that said, emerging research is beginning to show a decline in vaccine effectiveness overall. Based at least partly on that data — and the emergence of omicron — federal health authorities started recommending booster shots.

What else can people do to protect themselves?

Health experts say people who are not yet vaccinated should continue to wear masks and make plans to get vaccinated as soon as possible. Federal health authorities had been recommending masking in indoor public spaces, regardless of vaccination status. But as the number of covid-19 cases have dropped, the CDC has started relaxing certain recommendations — namely its mask recommendations. On Feb. 25, the CDC updated its mask guidelines , which no longer recommend masking for most people in areas with low to medium levels of covid-19 community transmission and hospitalizations . This includes indoor public spaces, including schools, or on school buses or vans in areas with low to medium infection levels, though individual students and staff members may choose to continue wearing masks. For those experiencing symptoms, experts say testing is key. Experts recommend getting tested at testing centers, pharmacies, urgent care centers or local health departments — but not emergency departments. Not only does this clog up emergency rooms, taking resources from those who have genuinely emergent illnesses or injuries, but it also creates a risk of exposure to covid-19 — both for the person unnecessarily visiting the ER and for others who are waiting for care, said William Schaffner, medical director of the National Foundation for Infectious Diseases. Regarding covid-19, he said emergency rooms should be reserved for those who are experiencing severe symptoms, such as trouble breathing. “You don’t want people to go to the emergency room with things that are not really emergencies. ERs are not clinics. They’re not set up for that,” he said.

What about people who are immunocompromised?

Vaccination is still the top recommendation to protect against the virus, 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. The CDC recommends that people with some immune system deficiencies — including those being treated for certain cancers, those who have received an organ transplant and those who have chronic medical conditions that can weaken immune responses — get a third primary shot of Pfizer-BioNTech or Moderna at least 28 days after completing the second dose in the series, and ultimately get a booster shot. Beyond vaccines, 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,” Schaffner 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 during influenza season, 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 disease.

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

Because delta was more contagious than the variants that preceded it, children were at somewhat greater risk of infection from it. But now, omicron is considered more transmissible, and although it does not appear to cause illness as severe as delta, public health authorities and pediatricians still recommend that eligible children get vaccinated. Children 5 years old and older are eligible for the Pfizer-BioNTech vaccine. The CDC signed off Nov. 2 on the vaccine for kids ages 5 to 11. Adolescents and teenagers were already eligible to receive the vaccine. Paul A. Offit, a pediatrician and the director of the Vaccine Education Center at Children’s Hospital of Philadelphia, suggests parents and guardians make sure that unvaccinated children ages 2 or older wear masks when they are in indoor public spaces, in accordance with CDC guidance . The CDC also recommends parents ensure 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.

How is the delta variant affecting the United States?

With more than half of the U.S. population fully vaccinated against the coronavirus, the infection fatality rate has declined. But over the summer of 2021 , the delta variant swept through communities where vaccination rates were lower, posing the most serious risk to those who are older, sicker and unvaccinated. By the end of the summer surge, the nation hit a tragic milestone — losing 1 in 500 people to the virus since the start of the pandemic. Although cases, hospitalizations and deaths from delta have significantly declined, the country saw another wave with omicron. Experts have said the coronavirus will never be gone but will evolve into an endemic disease such as influenza and other respiratory viruses.

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

There is not much research on this, but some people 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.

Who should get booster shots?

The CDC recommends booster shots for everyone 12 and older who is eligible to receive one. Those who completed an mRNA vaccine series — Pfizer-BioNTech or Moderna — are eligible for a booster five months after the initial series. Those 18 and older who took the Johnson & Johnson vaccine are eligible two months after the initial shot. The CDC added that people who are eligible for a booster can get any coronavirus vaccine authorized in the United States, regardless of which one they initially received. It other words, it is now acceptable to mix and match the vaccines. The decision was driven, at least in part, by data showing declining immunity from the vaccines to protect against less severe disease. “Around the six-month mark in the data, you start to see increases in mild to moderate infection,” U.S. Surgeon General Vivek H. Murthy said, adding that “the most important purpose of the vaccine is to keep us out of the hospital and to save our life.” The bottom line, experts said, is that people need to take measures to protect themselves and their loved ones against covid-19. Ben Guarino contributed to this report.

What you need to know about the highly contagious delta variant

The delta variant has been replaced by omicron as the dominant strain in the United States, but health experts say it’s still cause for concern.

By Lindsey Bever

Joel Achenbach

Kim Bellware

and

Lateshia Beachum

Updated March 14, 2022 at 8:22 a.m. EDT|Published September 16, 2021 at 12:48 p.m. EDT

A health-care worker fills a syringe with a dose of Pfizer's coronavirus vaccine in Los Angeles on Wednesday, Aug. 11. (Robyn Beck/AFP/Getty Images)

The delta variant of the coronavirus has been overtaken by omicron — now the dominant strain in the United States.

In the fall, delta had been accounting for more than 99 percent of new cases of covid-19. But delta cases have virtually disappeared as omicron and its mutations made up 100 percent of infections the week ending March 5, according to data from the Centers for Disease Control and Prevention.

What to know about the omicron variant of the coronavirus

Many health experts had believed that any future “variant of concern” was likely to be a descendant of the delta variant. But the newest variant, known by the Greek letter omicron, is not a delta offspring, coming from a different portion of the virus’s family tree. It was identified in November in southern Africa and cases have since been confirmed in dozens of countries — including the United States.

CDC Director Rochelle Walensky has said that all three coronavirus vaccines available in the United States can provide strong protection against severe disease and death from covid-19, the disease caused by the virus, including the delta variant. However, vaccine effectiveness declined somewhat as the delta variant was surging — and early research suggests vaccines are less effective against omicron as well. Some of that decline may be because of the natural waning of antibodies, or because of changes in behavior as people have been less likely to wear masks or engage in social distancing.

Here are answers to common questions about the delta variant.

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