Here’s How Well COVID-19 Vaccines Work Against the Delta Variant

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The United States is now dealing with the Delta variant of the coronavirus, a highly contagious variant that was first identified in India in December.

As with previous variants, the Delta variant has spread to many countries across the world, including, most notably, the United Kingdom, where it’s now responsible for around 99 percent of new cases.

The United States first announced it had a confirmed COVID-19 case with the Delta variant in March this year. It’s now the dominating variant nationwide, making up over 98 percent of all new infections in the country.

COVID-19 cases are quickly rising yet again in the United States. The average number of daily COVID-19 cases is more than 150,000, according to data from the Centers for Disease Control and Prevention (CDC).

This rise has been attributed, in part, to the Delta variant being an estimated 60 percentTrusted Source more transmissible than the alpha variant, according to research.

A recent study from the Chinese Academy of Medical Sciences has also found that “viral loads in Delta infections are [approximately] 1,000 times higher” than those caused by previous SARS-CoV-2 variants.

Additionally, areas with low vaccination rates have been more likely to see a surge in infections.

Written by Yasemin Nicola Sakay on August 26, 2021 — Fact checked by Dana K. Cassell for Healthline.com

The United States is now dealing with the Delta variant of the coronavirus, a highly contagious variant that was first identified in India in December.

This rise has been attributed, in part, to the Delta variant being an estimated 60 percentTrusted Source more transmissible than the alpha variant, according to research.

Additionally, areas with low vaccination rates have been more likely to see a surge in infections.

Low vaccine uptake driving up infections

“The unvaccinated population is at high risk for infection. If this variant continues to move quickly, especially in areas of low vaccination rates, the U.S. could see a surge in SARS-CoV-2 infection,” said Dr. Miriam Smith, chief of infectious disease at Long Island Jewish Forest Hills Teaching Hospital in Queens, New York City.

CDC Director Dr. Rochelle WalenskyTrusted Source issued a warning on this potential surge earlier this month.

In a press briefing, she said preliminary data suggested that 99.5 percent of the people who died from COVID-19 since January were unvaccinated.

“We know that the Delta variant… is currently surging in pockets of the country with low vaccination rates,” she said.

In Missouri, which has a vaccination rate of nearly 43 percent, confirmed coronavirus infections surged in July.

In contrast, Vermont currently has one of the highest vaccination rates in the country at 69 percent.

This echoes findings from a U.K. studyTrusted Source that found the Delta variant twice as likely to lead to hospitalization, and both the AstraZeneca-Oxford and Pfizer-BioNTech COVID-19 vaccines being effective in reducing this risk.

“We also know that our authorized vaccines prevent severe disease, hospitalization, and death from the Delta variant,” Walensky said.

Dr. Theodore Strange, the interim chair of medicine at Staten Island University Hospital in New York, said the data supports this.

“The safety and efficacy of the current vaccines are very clear. These three vaccines do work to prevent disease and the spread of disease, and they are as safe as any other vaccines that have been in use. Although some side effects have been reported, these issues are rare and treatable,” he told Healthline.

Vaccine progress in the U.S.

The United States has so far administered more than 430 million doses of COVID-19 vaccines.

More than 171 million people have received two doses. That means over 60 percent of the U.S. population over the age of 12 is fully vaccinated.

There are currently three COVID-19 vaccines that have been authorized for use by the Food and Drug Administration (FDA) and are currently in use across the United States.

The Pfizer-BioNTech vaccine was the first to receive full approvalTrusted Source and emergency use authorization (EUA)Trusted Source from the FDA. The vaccine is also available under EUA for those aged 12-15 and as a third dose in immunocompromised individuals. It is marketed under the brand name Comirnaty and is also referred to as BNT162b2. Currently, more than 205 million doses of the vaccine have been administered in the United States.

A week after the Pfizer-BioNTech vaccine EUA, the Moderna vaccine also got the green light from the FDATrusted Source. Sold under the brand name Spikevax and also known as mRNA-1273, more than 143 million doses of the Moderna vaccine have been administered in the United States so far.

The third vaccine approved by the FDA was the Johnson & Johnson COVID-19 vaccine, or Janssen COVID-19 vaccine. It was issued an EUA in February 2021Trusted Source. More than 14 million doses have been administered so far.

Vaccines vs. Delta variant

All three vaccines are proven to be effective in varying degrees against the original variant of the coronavirus, SARS-CoV-2, that causes COVID-19.

However, since the Delta variant emerged, scientists have been trying to establish whether these vaccines are as effective against it.

We broke down what the current data says. But new research could mean this data will change over time.

The Pfizer-BioNTech COVID-19 vaccine

Due to limited research so far, trying to determine the effectiveness of each vaccine against the Delta variant remains a challenge. However, there have been promising results from multiple studies.

Study 1 and real-life data

According to an analysis carried out by Public Health England, two doses of the Pfizer-BioNTech vaccine appeared to be about 88 percent effective against symptomatic disease and 96 percent effective against hospitalization with the Delta variant.

The same study suggested that the vaccine was approximately 80 percent effective against preventing infection from the Delta variant. Scientists came to this conclusion after analyzing 14,019 people with an infection, 166 of whom were hospitalized, in England.

Vaccines had a protective effect against infections with Delta and hospital cases were milder, the study found.

Public Health England also shared real-world data in May that solidified the importance of having a second dose of COVID-19 vaccine. The analysis suggested that a single dose of the Pfizer-BioNTech vaccine offered only about 33 percent protection against symptomatic disease.

This was a reduction from the previous 50 percent effectiveness estimated against the alpha variant.

The study also found that the Pfizer-BioNTech vaccine was 88 percent effective against the Delta variant 2 weeks after the second dose.

Study 2

A reportTrusted Source published in the journal Nature reflected the findings that a single shot of a two-dose vaccine such as Pfizer-BioNTech or AstraZeneca provided “barely” any protection.

However, researchers also reported that people who had received two doses of a vaccine had significantly more protection against infection with the Delta variant, with researchers estimating a level of 95 percent effectiveness.

The study also found that the Delta variant was less sensitive to “sera from naturally immunized individuals,” meaning people who had a prior infection may not be protected against reinfection with the Delta variant.

Study 3

A study in Canada, meanwhile, found that two doses of a COVID-19 vaccine worked just as well against the Delta variant as it did with alpha. It has not yet been peer reviewed.

The study suggested the Pfizer-BioNTech vaccine was around 87 percent effective 14 days after two doses.

Study 4

A study in ScotlandTrusted Source found similar results. The Pfizer-BioNTech vaccine, researchers concluded, offered “very good” protection against the Delta variant and demonstrated 79 percent effectiveness 14 days after receiving the second dose.

Study 5

A study in Israel was more of an outlier and found that the Pfizer-BioNTech vaccine did not offer as high protection as previously estimated. The study suggested the vaccine was about 64 percent effective against preventing infection with the Delta variant and 64 percent effective against symptomatic illness after two doses.

But scientists have pointed out that the full data has not yet been released, and it may have included asymptomatic infections picked up by Israel’s surveillance program.

Study 6

A study that analyzed vaccine effectiveness in India, where the Delta variant first emerged, found that the Pfizer-BioNTech vaccine was 88 percent effective against the Delta variant after two doses.

Effectiveness was considerably lower after one vaccine dose, however, at approximately 36 percent.

Study 7

A U.S. study, which has not yet been peer-reviewed, suggests that the effectiveness of the Pfizer vaccine has decreased against the Delta variant.

The data found that the vaccine offered 42 percent protection against symptomatic infection with the Delta variant in July, compared to over 76 percent in January.

The study also noted that those vaccinated with Pfizer were twice as likely to experience a “breakthrough infection”, compared to the Moderna vaccine.

Study 8

An Oxford University study on the impact of the delta variant on COVID-19 vaccine effectiveness found that two doses of either the Pfizer or AstraZeneca vaccine still offer significant protection against infection with the now prevalent variant but that this wanes within three months.

The study indicated that the efficacy of the Pfizer vaccine 90 days after the second dose dropped to 75 percent, compared to 85 percent two weeks after the second dose. However, this decline was more apparent in those older than 35 years of age.

The study has yet to be peer-reviewed.

Study 9

A studyTrusted Source, known as HEROES-RECOVER and published in the CDC’s Morbidity and Mortality Weekly Report, showed that vaccine effectiveness against preventing symptomatic and asymptomatic infection with the Delta variant fell to 66 percent from over 90 percent.

The real-world study tested over 4,000 U.S. frontline workers, more than 4 in 5 who were vaccinated with either the Pfizer-BioNTech or Moderna vaccines.

ESTIMATED EFFECTIVENESS OF PFIZER-BIONTECH VACCINE
Anywhere from 42 to 96 percent effective against the Delta variant with two doses.

Time for a COVID-19 vaccine booster?

In data released on July 28, Pfizer said the third dose of its COVID-19 vaccine “strongly” boosted protection against the Delta variant compared with its standard two-dose regimen.

According to the trial’s results, the third dose produced five times more antibodies against the Delta variant in people 18 to 55 years old and 11 times more antibodies in people 65 to 85 years old. The study is awaiting peer review.

The data has prompted Israel to become the first country to widely administer the third shot. Currently, people over 60 are being called in to book an appointment.

The Israeli Health Ministry said the decision also took into account its own data based on national health statistics, which suggested that people vaccinated in April appeared to have 75 percent protection against infection, according to reports, whereas protection for those vaccinated in January dropped to as low as 16 percent.

However, Smith told Healthline that the currently available vaccines have been widely effective in preventing severe disease, including those identified with current variants of concern, without boosters so far.

The CDC has so far only recommended a three-shot regimenTrusted Source for immunocompromised populations such as organ transplant recipients and cancer patients.

The Moderna COVID-19 vaccine

There are multiple lab studies that suggest the Moderna COVID-19 vaccine works against the Delta variant. And similar to the Pfizer-BioNTech vaccine, Moderna is also testing whether a third dose is beneficial.

Study 1

A lab study on the Moderna vaccine showed that the vaccine was capable of providing protection against the Delta variant and other variants tested, even though it was much more reduced compared with the alpha variant.

But the most interesting finding was that the vaccine was far more effective in producing antibodies against Delta than it was against beta, according to the data, which has not yet been peer-reviewed.

“As we seek to defeat the pandemic, it is imperative that we are proactive as the virus evolves. [T]hese new data are encouraging and reinforce our belief that the Moderna COVID-19 vaccine should remain protective against newly detected variants,” Stéphane Bancel, CEO of Moderna, said in a statement.

No clear level of effectiveness was mentioned.

Study 2

The same Canadian study that found the Pfizer-BioNTech vaccine to be 87 percent effective suggested that the Moderna vaccine was 72 percent effective against the Delta variant after one dose.

There wasn’t enough data to calculate protection after two doses for Moderna. It’s also important to point out that the study has not yet been peer reviewed.

The findings, however, indicate that even a single dose of the Moderna or Pfizer-BioNTech vaccines provides “good to excellent” protection against symptomatic infection as well as severe illness. Two doses were also found likely to provide even higher protection.

Study 3

A small laboratory study carried out by New York researchers suggests that the mRNA-based vaccines Pfizer and Moderna were both 94 to 95 percent effective in preventing COVID-19 with the Delta variant.

Study 4

A preprint study uploaded to medRxiv found that both the Moderna and Pfizer-BioNTech COVID-19 vaccines were highly effective at preventing infection in January, months before the Delta variant became dominant.

Both vaccines still appear to be highly effective at preventing death and severe illness with the Delta variant.

However, the newest data suggests that the vaccines’ effectiveness against symptomatic infection with the Delta variant dropped to around 76 percent for the Moderna vaccine and to as low as 42 percent for the Pfizer-BioNTech vaccine in July.

The study included more than 50,000 participants and was conducted in several U.S. states by the Mayo Clinic.

Venky Soundararajan, PhD, who led the study, said a Moderna booster shot may be recommended for people who got the Pfizer-BioNTech or Moderna vaccines earlier this year.

Study 5

Using data from the Office for National Statistics (ONS) COVID-19 Infection Survey, Oxford researchers compared the Pfizer, Moderna, and AstraZeneca vaccines against the delta variant and found that a single dose of the Moderna vaccine had similar or greater effectiveness compared to a single dose of the others.

However, the researchers said this could be due to the fact that those receiving the Moderna vaccine were younger on average and the vaccine had greater effectiveness in younger individuals.

Study 6

A studyTrusted Source examining the real-world performance of the Pfizer-BioNTech and Moderna vaccines among frontline workers found that effectiveness against infection dropped from 91 percent to 66 percent when the Delta variant became predominant in the United States in July 2021.

ESTIMATED EFFECTIVENESS OF MODERNA COVID-19 VACCINE
Studies mostly estimate a similar protection level as the Pfizer-BioNTech vaccine, with numbers ranging from 66 percent to 95 percent effectiveness.

The Johnson & Johnson COVID-19 vaccine

There’s little data that shows how effective the Johnson & Johnson (J&J) single-shot COVID-19 vaccine is at protecting against the Delta variant. The company is also reportedly researching whether a second shot would boost immunity against the variants.

Study 1

A recent clinical trial suggested that the vaccine was 85 percent effective against severe disease and demonstrated “strong, persistent” protection against hospitalization and death.

It also showed that the J&J vaccine prompted “neutralizing antibody activity,” which stops the virus from infecting healthy cells, against the Delta variant at a higher level than it did for the beta variant.

The Pfizer-BioNTech and AstraZeneca vaccines had shown a decline in effectiveness against the latter in another study.

Study 2

Interim results from a study involving 20 people have shown that the J&J vaccine neutralized the Delta variant within 29 days of the first shot, and protection improved over time.

“Current data for the eight months studied so far show that the single-shot Johnson & Johnson COVID-19 vaccine generates a strong neutralizing antibody response that does not wane; rather, we observe an improvement over time. In addition, we observe a persistent and particularly robust, durable cellular immune response,” said Dr. Mathai Mammen, the global head of Janssen Research & Development at Johnson & Johnson, in a July 1 press release.

Study 3

A study published on the preprint server BioRxiv suggested the J&J vaccine is 67 percent effective against the Delta variant.

The study also found the vaccine produced fewer antibodies against Delta than it did for the alpha variant, but scientists say the study only examined 27 people and may not accurately reflect its real-life performance.

The data has sparked discussions about a possible booster for the one-shot vaccine.

ESTIMATED EFFECTIVENESS OF J&J COVID-19 VACCINE
More comprehensive studies are needed to reach a definitive answer.

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Breakthrough infections can be expected

Although we are seeing more breakthrough cases in those vaccinated, and studies indicate that vaccine effectiveness against infection is waning slightlyTrusted Source over time, their protection against severe disease or death remains strong. Experts stress that vaccines are highly protective even if they’re not 100-percent effective.

A study that analyzed 161 vaccine breakthrough infections among 24,706 vaccinated healthcare workers found that although vaccinated people who contract SARS-CoV-2, the virus that causes COVID-19, have similar levels of the virus in their noses and throats as unvaccinated people, not all of it is as infectious.

The breakthrough cases presented with mild or asymptomatic infections, of which over 90 percent were attributed to the Delta variant.

Test-tube experiments showed that the viral shedding from vaccinated patients was lower, researchers reported. The study hypothesized that this was because some of the virus had already been neutralized by antibodies produced from the vaccine.

Key takeaways

The World Health Organization (WHO) has said data so far supportsTrusted Source claims that the Pfizer-BioNTech and AstraZeneca vaccines work against preventing severe COVID-19 caused by the Delta variant.

But it has also acknowledged that the vaccines may offer less protection against milder, symptomatic illness caused by Delta, though studies still suggest that people fully vaccinated “retain significant protection against the Delta variant.”

Receiving the full regimen of two doses of an mRNA COVID-19 vaccine, such as Pfizer-BioNTech or Moderna vaccines, has also shown to be much more effective against the Delta variant.

“The bottom line is that the vaccination program with any of the current vaccines available is the only way to break the cycle of spread by not allowing the virus to infect unvaccinated hosts and then mutate into variants such as Delta. These vaccines are safe and with a high degree of efficacy to prevent further morbidities and mortalities,” Strange said.

Prof. Tim Spector, an epidemiologist from King’s College London, told Healthline that it is now time for the United States to take lessons from the United Kingdom in dealing with this new variant.

“[They] should start spreading the word about the new symptoms. [D]o not get too relaxed when you get your vaccine either, especially if you are in a high-risk area,” he said.

“Your risk may be an eighth of what it was [after getting vaccinated] but still a considerable number of people will be infected,” he added, highlighting the importance of physical distancing and wearing masks in crowded, unventilated places.

Here’s How Well COVID-19 Vaccines Work Against the Delta Variant