Multiple studies show the FDA-authorized COVID-19 vaccines continue to be effective against the delta variant of the coronavirus, even if the potency of the vaccines is somewhat reduced. But a guest on Fox News falsely claimed the delta variant “really is not responsive at all, or protected at all by the vaccines” and there is “no clinical reason to go get vaccinated.”


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Update, Aug. 23: The Pfizer/BioNTech COVID-19 vaccine, which was previously authorized by the Food and Drug Administration for emergency use, received full approval from the agency on Aug. 23 for people 16 years of age and older.

The highly contagious delta variant of the coronavirus now accounts for 82% of new COVID-19 cases in the U.S., according to estimates by the Centers for Disease Control and Prevention, and is partly responsible for the rising number of infections across the vast majority of the country.

Data show that while the authorized COVID-19 vaccines may be less protective against delta in preventing infections and symptomatic illness compared with earlier versions of SARS-CoV-2, they still largely remain effective — and are very good at preventing serious disease and death.

As CDC Director Dr. Rochelle Walensky said in a July 16 press briefing, “This is becoming a pandemic of the unvaccinated,” as outbreaks continue in places with low vaccination rates.

But that’s not the message Fox News guest Dr. Peter McCullough has been spreading. In a July 13 interview on “The Ingraham Angle,” the private practice internist played down the risks of the delta variant and falsely said there was “no clinical reason to go get vaccinated.”

“We are at a very low baseline now,” he said in reply to host Laura Ingraham, who asked him what to expect next from the pandemic. “We are going to have a slight rise with the delta variant, but the delta variant really is not responsive at all, or protected at all by the vaccines. So 42% of 90,000 proven delta cases in the U.K. have been vaccinated. In Israel, the estimate right now from the Israel health authorities that the vaccine efficacy rate is only about 60 to 70%. So we expect delta cases and in fully vaccinated individuals. It will be a mild rise. It’s easily treatable in high-risk patients. There is no reason right now, no clinical reason to go get vaccinated.”

As we’ll explain, McCullough, who has previously been a source of COVID-19 misinformation, including about vaccines, is correct about the Israeli percentage, but wrong about the U.K. one — and there is an abundance of data contradicting his claim that the vaccines do nothing against the delta variant.

As for the idea that COVID-19 is “easily treatable,” there is little to support that notion as well, especially with hundreds of Americans still dying every day from the disease.

“You’re much, much better off being vaccinated than not,” Frederic Bushman, co-director of the University of Pennsylvania’s Center for Research on Coronaviruses and Other Emerging Pathogens, told us. “The delta variant may reduce the effectiveness [of the vaccines] a little, but still they’re so effective that you get a lot of benefit.”

COVID-19 Vaccine Effectiveness Against Delta

Scientists don’t have information from the clinical trials about how well the FDA-authorized COVID-19 vaccines perform against the delta variant because the phase 3 trials were done at a time before the variant was circulating widely. The variant, also known as B.1.617.2, was first identified in India in October 2020 and is more than twice as transmissible as the original versions of the coronavirus, according to the CDC. Some data also suggest delta may cause more severe disease, but this has not been conclusively established.

There are, however, multiple studies of how the vaccines are faring in the real-world against delta, and most show the vaccines are working largely as expected.

study published in the New England Journal of Medicine on July 21 by Public Health England, for example, found that after the two recommended doses, the effectiveness of the Pfizer/BioNTech vaccine in preventing symptomatic disease in the U.K. fell only slightly, to 88.0% against delta from 93.7% against the alpha variant — another more contagious version of the virus, known as B.1.1.7, that previously predominated in the country. The authors, however, noted that a larger decline and lower effectiveness was observed after a single dose, underscoring the importance of receiving both immunizations.

In an unpublished study that has not yet been peer reviewed, the U.K. health agency also found the Pfizer/BioNTech vaccine was very effective at preventing hospitalization with the delta variant, reducing the risk by 94% after one dose and by 96% after two.

Other countries have reported similar preliminary results. In an unpublished study from Ontario, Canada, researchers estimated the effectiveness of the Pfizer/BioNTech vaccine against symptomatic infection with delta to be 87% after two doses, while a report from Scotland, published in the British medical journal the Lancet, pegged the figure at 79% for infection with delta.

The Canadian report also found that one dose of the Moderna vaccine was 70% effective in preventing symptomatic disease and 95% effective in preventing hospitalization or death; there were too few COVID-19 cases among the vaccinated to estimate the effectiveness after two doses.

Meanwhile, in Israel, the health ministry released preliminary, unpublished information on July 5 suggesting that with the rise of the delta variant in the nation, the Pfizer/BioNTech vaccine was now 64% effective against infection or symptomatic illness, but still 93% effective in preventing serious illness and hospitalization. The ministry had previously estimated the vaccine to be 97% effective against symptomatic disease, with similar levels of protection against hospitalization and death, at a time when the alpha variant made up the vast majority of COVID-19 cases.

McCullough, therefore, is right that Israel estimated the Pfizer/BioNTech vaccine effectiveness at “only about 60 to 70%.” But that is still a substantial amount of protection — and McCullough neglects to mention the vaccine’s excellent ability to prevent the worst outcomes of COVID-19.

In late July, the ministry further reduced its vaccine effectiveness estimate for symptomatic COVID-19 to 40.5%, but once again, found protection against severe disease to remain robust — 88.0% effective against hospitalization and 91.4% effective against severe COVID-19.

It’s worth mentioning that some experts doubt the accuracy of the Israeli estimates, which are outliers compared with the other results and rely on a different methodology. In its July 27 scientific brief on the vaccines, the CDC said “more technical information is needed to allow full interpretation.”

The most recent Israeli figure, in particular, is based on a small number of cases over a short period of time and should be considered preliminary, according to an expert advising the Israeli government on the coronavirus. The ministry itself also acknowledged the results might be skewed because of differential testing among the vaccinated and unvaccinated populations.

Calculating vaccine efficacy, which is the metric used in clinical trials, is relatively straightforward and less prone to bias because the groups getting vaccinated or a placebo are randomized, although it cannot capture certain aspects of the vaccine’s performance under real-world conditions, such as changes in the virus, imperfect vaccine storage conditions, or how well certain individuals might respond, as the CDC explains.

In contrast, estimating effectiveness — the term used for observational studies that get at the effect of the vaccines in the real world — is more difficult because other features about those who are vaccinated or not might influence the likelihood of infection or illness.

Several experts pointed us to the U.K. study as the most robust to date, although more data are needed to better understand how well the vaccines work against delta, and things could change in the future.

“I would say the best document is the New England Journal of Medicine paper,” said Dr. Peter Hotez, a vaccine expert and the dean of the National School of Tropical Medicine at Baylor College of Medicine. “I think that looks pretty sound, and it makes sense,” he said, that the effectiveness of the Pfizer/BioNTech vaccine might go down slightly for symptomatic illness, but still hold up well against severe disease.

“Those data seem consistent with the USA experience so far,” said Shane Crotty, an immunologist at the La Jolla Institute for Immunology who has been studying COVID-19, of the U.K. paper, in an email. “Most hospitalization[s] are of unvaccinated people, and most of the outbreaks are in low vaccination states so far.”

Regardless of the specifics, McCullough’s attempt to use Israeli and U.K. data to argue against immunization contradicts the conclusions and calculations of the health agencies in those countries.

As we already said, Israel’s health ministry has consistently found vaccination results in a more than 90% reduction in the risk of severe disease, even against delta. The ministry also announced on July 25 that a government analysis found that “vaccinated individuals with pre-existing conditions are better protected against serious illness compared to non-vaccinated individuals without any risk factors.”

And Public Health England, which has largely relied on the AstraZeneca vaccine, estimates that full vaccination lowers the risk of symptomatic disease by 79% and the risk of severe illness by 96% against the delta variant. Vaccination is highly recommended in both nations.

Lab Studies of Vaccine Immune Responses

The other set of information about how well the vaccines might do against delta or other variants comes from lab tests assessing the immune responses in those who have been vaccinated.

One common experiment is to take blood samples from vaccinated volunteers and test whether the antibodies present in the sera can still neutralize SARS-CoV-2 variants or viruses engineered to have the variant spike proteins.

Many of these types of studies show that most fully vaccinated people produce so-called neutralizing antibodies that work against delta, but at a lower level than with ancestral versions of the coronavirus — and in negligible amounts if immunized with only one dose of the two-dose vaccines.

One study, for example, published on July 8 in Nature as an unedited manuscript by researchers in France, found that 94% of people fully vaccinated with the Pfizer/BioNTech vaccine produced antibodies that could neutralize the delta virus, but only 13% of people did the same after one dose, with similar results for the AstraZeneca vaccine, which has been widely used in the U.K.

A similar paper published in Nature in June with support from Pfizer and BioNTech found that neutralization of the delta variant by serum from people immunized with that vaccine was “only modestly reduced” compared with the original virus.

Multiple other studies have identified small or moderate decreases in neutralization against delta after vaccination with the Pfizer/BioNTech or Moderna vaccines, suggesting that they retain most of their effectiveness against the variant.

Less information is available about the Johnson & Johnson vaccine, although the company announced on July 1 that two small studies, one of which is now published in the New England Journal of Medicine, show that vaccinated people produce antibodies that can neutralize delta and that they have long-lasting T cell responses.

More recently, an unpublished study from NYU Grossman School of Medicine found significantly lower neutralizing antibody activity against delta in people vaccinated with the J&J vaccine compared with the two mRNA vaccines, raising the concern that the single dose vaccine might need a booster.

But experts say it’s hard to make firm conclusions based on that data. Baylor’s Hotez, for example, said there’s “still a big leap” between lab tests and how well the vaccines perform in people. Importantly, he said, neutralizing antibody levels were also lower against the beta variant that was first detected in South Africa (B.1.351), but clinical trials demonstrated the J&J vaccine was still effective there, albeit to a lesser degree. That makes it likely, he said, that the J&J vaccine provides “some level of protection” against delta. For its part, J&J notes that in its studies, antibody levels against delta are higher than those against beta.

Dr. Eric Topol, a professor of molecular medicine at Scripps Research, said on Twitter that the results of the two antibody studies weren’t actually contradictory, as there were still detectable levels of neutralizing antibodies in the NYU preprint.

Indeed, part of the problem is that scientists have not yet fully identified so-called correlates of protection, or immune markers that can be used to know whether a vaccine will be protective, such as what antibody titer, or level, is needed to prevent infection or disease.

It’s worth noting, too, that despite the emphasis on antibody levels, which are relatively easy to check in the blood, there are also the already primed antibody-producing B cells and T cells that can quickly spring into action and limit an infection if a vaccinated person is exposed to the coronavirus.

Coronavirus Infections Expected Among the Vaccinated

As part of his argument that the COVID-19 vaccines don’t work against delta, McCullough claimed that “42% of 90,000 proven delta cases in the U.K. have been vaccinated.”

We asked him for the source of this statement, and he told us it came from the U.K. government’s technical briefing on the variants from June 25. But the report actually shows that only 7,235 people out of 92,029 delta cases, or 7.9%, were fully vaccinated, while 58% were unvaccinated and the remainder were partially vaccinated or did not have vaccination information. By inaccurately citing 42%, McCullough gives the false impression that the vaccines are failing on a wide scale against delta.

More important, experts say it can be misleading to look at these kinds of percentages without the larger context. That’s because no vaccine is 100% effective. So as vaccination rates go up, more and more of the people who get COVID-19 will have been immunized, but fewer total people will get the disease.

“As more folks get #vaccinated, the % of folks who get sick with #COVID19 & happen to be vaccinated will increase. This is expected,” explained Maia Majumder, a computational epidemiologist at Boston Children’s Hospital, on Twitter. “[I]t doesn’t mean that #vaccines aren’t working; rather, it reflects the realities of probability.”

She pointed to a handy graphic by the Financial Times that does the math and shows how it’s possible to have 40% of hospital admissions be among vaccinated people, even if the vaccine is 80% effective.

The U.K. and Israel have some of the highest vaccination rates in the world, so their data can be easy to misinterpret.

For now, similar percentages in the U.S. remain compelling. According to the CDC, less than 3% of COVID-19 hospitalizations between January and May were in fully vaccinated people. CDC Director Dr. Rochelle Walensky also has said that preliminary data from a collection of states for the first half of 2021 suggest that 99.5% of COVID-19 deaths have been in the unvaccinated.

But these numbers will change seemingly for the worse, even if vaccine performance holds. The numbers that people should really pay attention to, says Wake Forest University statistician Lucy D’Agostino McGowan, are the fractions of the vaccinated and unvaccinated who die or are hospitalized because of COVID-19. As she explains in a blog post, it’s the comparison of those two numbers that gives an accurate view of how well the vaccines work.

So far, the CDC says it has not observed a disproportionate number of so-called “breakthrough” cases — instances of infection or disease despite vaccination — from any variant, although it switched to monitoring only breakthrough hospitalizations and deaths in May.

The agency noted in its scientific brief on vaccination that an unpublished study from Houston that has yet to be peer reviewed found that “Delta caused a significantly higher rate of breakthrough infections in fully vaccinated people compared with infections from other variants.” That paper also found that only 6.5% of all COVID-19 cases were in fully vaccinated people and few of those cases required hospitalization.

Indeed, the most important message, experts and public health officials say, is that even if the available vaccines have diminished effectiveness against delta in preventing infection and mild disease, they remain highly effective against the worst outcomes and are a critical tool for reigning in the pandemic.

“The vaccines still work really well, they just don’t work quite as well,” Penn’s Bushman said. “You’re much less likely to get infected. If you get infected, you’re much less likely to get seriously ill.”

A CDC study published on July 22 in the New England Journal of Medicine found that for both the Pfizer/BioNTech and Moderna vaccines, vaccination helped reduce the severity of COVID-19 in a group of health care workers when they were infected, shortening the illness and cutting the chance of having a fever.

An unpublished study from India, posted to the preprint server medRxiv on July 16, similarly found that full immunization with an Indian-manufactured AstraZeneca vaccine or the inactivated vaccine from India’s Bharat Biotech was associated with a 50% lower risk of death among hospitalized COVID-19 patients, nearly all of whom had been sickened with the delta variant. Vaccinated patients also had less severe disease, even though they were older and had more risk factors.

The chance of having a breakthrough infection appears to go up with age and is more likely in people who are immunocompromised, according to studies reviewed by the CDC. That fits with what’s expected, and could lead the agency to recommend booster shots for certain populations, which the CDC’s vaccine advisory committee has started to discuss.

Update, Aug. 13: The Food and Drug Administration authorized a third dose of the Pfizer/BioNTech and Moderna vaccines for certain people with weakened immune systems on Aug. 12, with the CDC’s vaccine advisory committee and the agency endorsing that decision the following day. The CDC recommends an extra dose only for people who are “moderately to severely” immunocompromised, such as organ transplant recipients or those undergoing cancer treatment. 

One concern with delta is that if vaccinated people do become infected with the variant, they may still be able to transmit the virus to others. The CDC has said this is suggested by data showing that the amount of virus in vaccinated people infected with delta is approximately the same as those who weren’t vaccinated.

The agency cited the new evidence in its rationale to change its mask recommendations on July 27 to have fully vaccinated people wear masks indoors in areas with “substantial” or “high” transmission. Vaccinated people, however, would still be less likely to spread the virus than unvaccinated people because they’re less likely to be infected in the first place. (See our SciCheck article, “A Guide to the CDC’s Updated Mask Recommendation.”)

It’s worth noting that not all breakthrough infections are necessarily an entirely bad thing — provided they are mild and don’t lead to additional spread of the virus  — as they could act as a kind of “booster” to the vaccine, reinforcing the body’s existing immune response against the virus and strengthening it. But of course, vaccinated people should not seek out infections, as these can still be risky and can include persistent symptoms of “long COVID-19.”

Although there is a range of views, many experts say that while the vaccines retain the ability to protect against serious disease, there is little reason for most people to get an additional immunization.

“I think what we’re seeing in data that’s being generated in different parts of the world, even around the delta variant, is that these vaccines are very highly protective against severe COVID, hospitalization and death,” said Dr. Anna P. Durbin of Johns Hopkins University’s Center for Immunization Research in the July 19 edition of the school’s “Public Health On Call” podcast. “The best way that we can control COVID and control the generation of these new variants and variants of concern is to get as many people in the world vaccinated as possible. So, right now, based on the data that we have, to me, it makes far more sense — far greater sense — to provide the rest of the world with as many doses of vaccines as we possibly can.”

Bushman agreed. “Ethically, it’s the right thing to do, trying to roll out vaccines to places that don’t have it,” he said. “And from a purely self-serving view, I think it’s also the most effective course of action to protect ourselves.”

Durbin, however, cautioned that this could change. Hotez similarly said that he thinks it’s likely that eventually an extra shot will be needed, but not necessarily for the delta variant.

COVID-19 Not ‘Easily Treatable’

Part of McCullough’s argument for why vaccines aren’t necessary is that COVID-19 is already “easily treatable in high-risk patients.” But there remain few treatments that have demonstrated any benefit to COVID-19 patients and none that render treatment easy.

In response to an inquiry from, McCullough further claimed that because early treatment “modifies the severity and risk of hospitalization and death of the disease,” statements about vaccines reducing disease severity “would have to account for early treatment.” He cited several studies advocating unproven drugs as part of that early treatment protocol. “To my knowledge no such data exists to support any claim that in addition to early treatment the vaccine has any effect on outcomes,” he added.

But Caitlin Rivers, an epidemiologist at Johns Hopkins Bloomberg School of Public Health, told us that his logic was flawed.

“For early treatment to confound outcomes in people who are infected, it would have to be offered only to people who are vaccinated,” Rivers said in an email to “But that’s clearly not the case – people who are unvaccinated have the same access to medical care. The evidence that vaccines protect against severe illness is very strong.”

Moreover, McCullough’s version of early treatment is with a combination of therapies that includes several drugs that the National Institutes of Health recommends against using, including hydroxychloroquine with azithromycin. Another of the drugs, dexamethasone, has been shown to help COVID-19 patients, but only in specific circumstances in hospitalized patients — not during early treatment, which the NIH advises against.

The only drugs the NIH recommends using early are some of the synthetic monoclonal antibodies that target SARS-CoV-2, which are designed to prevent the virus from entering cells, and are for high-risk patients only. But again, there is no reason to think that vaccinated and unvaccinated people would have differential access to those treatments.

Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over our editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.


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A healthcare worker prepares a dose of the Pfizer-BioNTech Covid-19 vaccine during a vaccination event at a school in Boston, Massachusetts, U.S., on Monday, August 16, 2021. With the highly contagious Delta variant sending more children to hospitals, a dozen states now require K-12 students to wear masks, while another handful bar such mandates. Photographer: Allison Dinner/Bloomberg via Getty Images