The Delta Variant and Covid-19 Vaccines: What to Know - The Wall Street Journal

Almost everyone hospitalized with Covid-19 in recent weeks has been unvaccinated, doctors and public officials have said.

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The Delta variant of the Covid-19 virus, the dominant strain in the U.S., is driving up cases in particular where vaccination rates are lower.

Also known as B.1.617.2, the variant was first identified in India in late 2020. It has spread to at least 85 countries, according to the World Health Organization, and has itself mutated to produce a further version that is causing concern in India.

Vaccines from Moderna Inc., Johnson & Johnson and Pfizer Inc. and partner BioNTech SE—the three shots authorized in the U.S.—were primarily tested in large clinical trials for their efficacy at preventing symptomatic disease, rather than their ability to prevent infection completely. While the vaccines seem to be slightly less effective at preventing symptomatic Covid-19 from the Delta variant, research has shown that they remain highly effective at preventing serious illness and hospitalization.

Overall, the portion of eligible Americans fully vaccinated against Covid-19 has been stalled for weeks at around 57%.

Here is what we know so far about the Delta variant and vaccines.

What is the Delta coronavirus variant?

The B.1.617 lineage, identified in October, refers to several variants of the virus that causes Covid-19.

The most concerning of the lineage is the Delta, or B.1.617.2, variant, which scientists say appears to have two advantages over earlier forms of the virus. It has been established to be more infectious, and it also appears to be more effective at evading vaccines, though people who are fully vaccinated still have significant protection against illness.

What are the symptoms of the Delta variant?

In general, they are the same as the symptoms of other Covid-19 strains: fever, dry cough, shortness of breath, temporary loss of taste or smell, flulike respiratory and digestive problems, muscle aches and fatigue.

There is no data yet to show that the variant is causing atypical symptoms. There were reports in India suggesting that the virus was causing hearing loss, gangrene and other unusual symptoms, but scientists said an intense epidemic such as the one in India will tend to produce a spectrum of rare conditions, given the large number of infections.

Covid-19's Delta variant is proliferating world-wide threatening unvaccinated populations and economic recovery. WSJ breaks down events in key countries to explain why Delta spreads faster than previously detected strains. Composite: Sharon Shi The Wall Street Journal Interactive Edition

How widespread is the variant in the U.S.?

More widespread every day. Delta now accounts for 83% of known cases in the country, said Centers for Disease Control and Prevention Director Rochelle Walensky.

Its spread is contributing to higher case counts, hospitalizations and deaths attributed to Covid-19. Almost everyone hospitalized with Covid-19 in recent weeks has been unvaccinated, doctors and public officials have said.

How contagious is this variant?

Scientists are still studying the virus, and their early conclusions aren't definitive. British scientists, who have probably done the most work on the variant, estimate it is from 40% to as much as 80% more infectious than the so-called Alpha variant, or B.1.1.7, which was first identified in England last year, is now prevalent in the U.S. and is itself more contagious than the version of the virus that emerged in China in 2019.

Researchers in China reported in July that people infected with the Delta variant had about 1,000 times more viral particles in their respiratory tracts than those with the original strain. The researchers also reported that people become infectious sooner.

Wendy Barclay, professor of virology at Imperial College London, said swab tests suggest that Delta infects people with a heavier virus load, which means they exhale more of it for others to catch. The mutations also appear to make the variant more effective at attaching itself to cells in human airways. The combination means that an infected person is, other things being equal, likely to infect more people and that people require less exposure to become infected.

Scientists don't yet know if the Delta variant is deadlier than other variants, but a study published in the Lancet medical journal estimated that the risk of hospitalization after infection with Delta could be 85% higher than with Alpha.

Will existing vaccines work against this variant?

Designed to target the earliest coronavirus version, Covid-19 vaccines are holding up well against the Delta variant because it isn't so different from the original strain that it can evade the shots, researchers say. The vaccines generate neutralizing antibodies, which prevent the virus from entering cells and replicating. The number of antibodies generated in fully vaccinated people is still so high that enough antibodies can bind to the spike protein and block the virus, researchers say.

No vaccine is 100% effective. While vaccines in use in the U.S. and Europe appear to work well in preventing symptomatic Covid-19 and serious illness, research shows, they appear to work slightly less well than against earlier versions of the virus.

Vaccines from Pfizer and BioNTech as well as from AstraZeneca PLC were highly effective against the Delta variant after two doses, according to a study of nearly 20,000 people published in July in the New England Journal of Medicine.

After two doses, the Pfizer vaccine was 88% effective at preventing symptomatic disease from Delta, compared with 36% efficacy after one dose, said the study, which was sponsored by Public Health England. The effectiveness at preventing symptomatic disease after two doses of AstraZeneca's shot was 67%, compared with 30% after one dose.

A separate, preliminary study, however, found a much lower rate of efficacy against the variant for the Pfizer-BioNTech shots. Data released in late July by the government of Israel, which had one of the world's fastest and most comprehensive vaccine rollouts, indicated that a two-dose course of the Pfizer-BioNTech vaccine is 41% effective in preventing symptomatic disease from the Delta variant. The same Israeli study, based on a small sample size, found that the two-dose Pfizer vaccine is 88% effective at preventing hospitalization and 91% effective at preventing severe Covid-19 caused by the Delta variant.

Two doses of the mRNA vaccines are needed, researchers say, because neutralizing antibodies don't work as well after just one dose and the body needs a higher level of antibodies given the presence of mutations. Also, the greater levels of neutralizing antibodies triggered by the two doses create a stronger immune response, virologists say. A second dose boosts the numbers of T-cells, which hunt down infected cells and destroy them, and memory B-cells, which circulate in the blood and help churn out antibodies upon detecting a virus.

Moderna has said lab tests on blood samples taken from some vaccinated people showed promising signs that the company's Covid-19 shot would protect against Delta and other variants. Johnson & Johnson has said that its shot showed promising signs in a small lab study of protecting against Covid-19.

There is no evidence that young adults and children are more at risk proportionately from this variant than other age groups, and the increased transmission mostly reflects that they haven't been immunized, scientists say.

Should I wear a mask even if I'm vaccinated to protect against Delta?

The CDC has said vaccinated people should wear a mask to prevent transmission of the virus in enclosed public places. The agency also said everyone in schools should wear masks this fall, regardless of vaccination status.

The agency said wider mask-wearing would help prevent a vaccinated person with a breakthrough infection from spreading the virus to the unvaccinated. Some doctors say that because Delta can generate a heavier viral load than earlier variants, raising the risk of infection, masks can provide an added layer of protection on top of vaccination under conditions where Delta may be circulating, such as close quarters in a group of people whose vaccination status is unknown.

"You need to be prudent even if vaccinated," said Dr. Diego Hijano, a pediatric infectious-disease specialist at St. Jude Children's Research Hospital in Memphis, Tenn.

Can I get infected with this variant even after I've had Covid-19 or been vaccinated against it?

Yes, it is possible to become reinfected, though there is some evidence that second infections are on average less severe than the first. Virologists and infectious-disease specialists say that neutralizing antibodies, T-cells and memory B-cells generated by the vaccines are likely helping prevent severe disease. T-cells recognize viral cells in more ways than neutralizing antibodies and likely retain strength against variants, they said.

"If you're vaccinated, you should not worry about the Delta variant," says Carlos del Rio, an infectious-diseases doctor and executive associate dean of Emory University School of Medicine in Atlanta. "If you're not vaccinated, you are really in trouble because it's likely you will get infected."

If you are vaccinated but become infected with the Delta variant, how likely are you to spread the virus?

Doctors and researchers say that this is one of the most urgent questions about Delta, and that unfortunately we don't have a definitive answer; there isn't enough data yet.

Many researchers say vaccinated people who get a breakthrough infection, including via the Delta variant, are less likely to transmit Covid-19 to others. That's because in most cases for vaccinated people breakthrough infection will result in a strong immune response that will quickly control and clear the infection. This, in turn, limits the amount of infectious virus being shed by the vaccinated.

"Vaccine or not, you're infected," said Gabor Kelen, director of the Department of Emergency Medicine at Johns Hopkins Medicine. "But the hope is that your viral load is significantly less than it would be if you weren't vaccinated."

Factors affecting transmission include the viral load of the infected person but also the duration of exposure to another person and proximity of the two individuals, he said.

In a recent study of about 4,000 healthcare workers, people who became infected with Covid-19 after getting an mRNA vaccine carried less virus and had shorter cases than unvaccinated people who became infected. Researchers took samples from study participants before the Delta variant is believed to have arrived in the U.S.

However, researchers say that because Delta is so infectious it may be more transmissible than previous lineages and able to overcome the factor of lower viral shedding. Doctors and virologists say that vaccinated people are likely shedding less virus after getting a shot, but perhaps not as little as with other variants because Delta is known to be more contagious in general.

Daniel Kuritzkes, the chief of the infectious-diseases division at Brigham and Women's Hospital, said there are no confirmed cases of vaccinated individuals becoming infected and passing the virus along to unvaccinated individuals. He said the main setting where this could be a concern is households where some family members are unvaccinated, such as children or adults who are immunocompromised.

He said sustained close contact could potentially result in transmission despite the low levels of virus in the breakthrough cases, but is unlikely to happen, Dr. Kuritzkes said. "We think it's very unlikely that vaccinated people who come down with Covid will spread the virus to others, regardless of variant," he said.

How is Delta affecting the course of the pandemic outside the U.S.?

Delta played a significant role in a record-setting surge of infections that overwhelmed India's healthcare system. It is the dominant variant in the U.K., accounting for 97% of samples sent for genetic analysis in mid-June. After cases of the variant surged in the U.K., Prime Minister Boris Johnson delayed by four weeks the country's plans to drop all remaining Covid-19 restrictions.

The emergence of the variant in Israel, Australia and elsewhere has led governments to reimpose restrictions. It is also driving up infections in many other parts of the world, including Africa, which is largely unvaccinated. The World Health Organization says it has been detected in 21 African countries.

Write to Jared S. Hopkins at jared.hopkins@wsj.com and Robbie Whelan at robbie.whelan@wsj.com

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