The Delta variant of SARS-CoV-2, the virus that causes COVID-19, is more transmissible than preexisting variants, and it has rapidly become the dominant variant in several countries, including India and the United Kingdom. Some reports suggest that existing COVID-19 vaccines may be less effective in preventing infection with Delta. Can additional booster shots help?

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Can vaccine boosters help keep the Delta variant at bay? Here’s what we know so far. Image credit: Eva Marie Uzcategui/Bloomberg via Getty Images

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Over the past few months, the Delta variant of SARS-CoV-2 has spread widely in countries around the world, becoming the dominant variant in many places.

Its rapid spread has recently led countries, such as Australia, to reinforce strict lockdowns, as emerging data suggest the variant is more infectious than preexisting ones, such as the Beta variant, and that it may be able to bypass existing COVID-19 vaccines in some cases.

Prof. Sir Andrew Pollard, head of the Oxford Vaccine Group, which has contributed to the development of the Oxford-AstraZeneca vaccine, has even commented that, in his opinion, the highly transmissible Delta variant has made achieving herd immunity an impossibility.

“The Delta variant will still infect people who have been vaccinated. And that does mean that anyone who’s still unvaccinated at some point will meet the virus […], and we don’t have anything that will [completely] stop that transmission,” he told The Guardian.

Additionally, recent data have also suggested the immunity provided by COVID-19 vaccines fades considerably over time, which also means vaccinated individuals become more susceptible to infection with SARS-CoV-2.

However, some scientists and pharmaceutical companies argue that offering an additional booster shot of some of the most widely authorized COVID-19 vaccines could provide an effective way to keep the Delta variant at bay.

But what does the evidence say so far, and how are countries worldwide responding to the notion of incorporating additional booster shots in their COVID-19 vaccination campaigns?

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While published data on the effectiveness of COVID-19 vaccine boosters against the Delta variant are not yet available, some of the pharmaceutical companies that produce and distribute COVID-19 vaccines have announced that recent clinical trials support this perspective.

According to Pfizer’s 2021 second-quarter earnings report, receiving an additional booster dose of their COVID-19 vaccine after having had the initial two doses increases the amount of Delta variant antibodies fivefold in 18-to-55-year-olds and 11-fold in 65–85-year-olds.

In answer to queries from Medical News Today, a Pfizer spokesperson explained that this “conclusion is based on initial data from the ongoing booster trial of a third dose of the current BNT162b2 vaccine and laboratory tests.”

“The booster trial builds on the phase 1/2/3 trial and is part of the companies’ clinical development strategy to determine the effectiveness of a third dose against evolving variants,” they noted, adding that Pfizer “expect[s] to publish more definitive data about the analysis in the coming weeks.”

This third dose would be identical to the two doses of the currently authorized Pfizer vaccine. However, the company is also investigating how an “updated” vaccine dose, altered to target the Delta variant specifically, would fare.

A Pfizer spokesperson told MNT:

“The ongoing booster trial is evaluating the safety and tolerability of the current BNT162b2 vaccine. While we believe a third dose of BNT162b2 has the potential to preserve the highest levels of protective efficacy against all currently known variants, including Delta, we are remaining vigilant and are also developing an updated version of the vaccine that targets the full spike protein of the Delta variant. The first batch of the mRNA for the trial has already been manufactured, and we anticipate the clinical studies to begin in August, subject to regulatory approvals.”

Moderna has also said that an additional booster shot of its COVID-19 vaccine would be able to keep the Delta variant at bay.

The company made this announcement, initially, in its own second-quarter financial report, which states that “[r]obust antibody responses have been observed from existing Moderna booster candidates against COVID-19 in phase 2 studies.”

“In a phase 2 study, vaccination with 50 [micrograms] of three different Moderna mRNA booster candidates induced robust antibody responses […] against important variants of concern, including Gamma (P.1); Beta (B.1.351); and Delta (B.1.617.2),” the report also states.

The three boosters under investigation included their currently authorized shot, as well as two more experimental candidates.

According to Moderna’s report, the levels of neutralizing antibody generated after the third booster shot were similar to those registered after two 100 microgram doses of their currently authorized vaccine.

Following these findings, both Pfizer and Moderna have been seeking authorization for their respective booster shots from countries that have already authorized their main COVID-19 vaccines.

So far, the Food and Drug Administration (FDA) has authorized the distribution of third booster shots of both the Pfizer and the Moderna COVID-19 vaccines — but only for those who are immunocompromised, and therefore at higher risk of infection with emerging variants of SARS-CoV-2.

Israel has also recently authorized the distribution of third shots of the Pfizer vaccine, which is now available to “people over 50, healthcare workers, people with severe risk factors for the coronavirus, [and] prisoners and wardens.”

While the United Kingdom has not yet authorized additional booster shots, unofficial reports indicate it has ordered millions of extra doses for a COVID-19 vaccine booster campaign in fall 2022.

“We have secured access to more than 500 [million] doses of COVID-19 vaccines, and we are confident our supply will support potential booster programs in the future. The potential booster program will be based on the final advice of the independent [Joint Committee on Vaccination and Immunisation],” the Department of Health and Social Care told The Guardian.

While booster shots may go some way towards offering better protection against emerging variants of SARS-CoV-2, the World Health Organization (WHO) has expressed concern that the rapid distribution of third vaccine doses in high income countries will further contribute to vaccine nationalism and widen the vaccination gap in low and middle income countries.

“I understand the concern of all governments to protect their people from the Delta variant. But we cannot accept countries that have already used most of the global supply of vaccines using even more of it,” said Dr. Tedros Adhanom Ghebreyesus, WHO Director-General, in a press conference on August 4.

In an effort to try and break this ever-widening gap, the WHO has asked high income countries at the forefront of vaccine development and distribution not to offer any additional booster doses until September 2021, at least.

“We need an urgent reversal from the majority of vaccines going to high income countries to the majority going to low income countries,” Dr. Tedros emphasized.

Speaking to Nature in a news article, Dr. Laith Jamal Abu-Raddad, an infectious disease epidemiologist at Weill Cornell Medicine–Qatar in Doha, argues the benefits of offering additional booster doses to those already fully vaccinated against COVID-19 are minimal.

Dr. Abu-Raddad claims this would amount to “[w]asting resources on boosters for those who are already protected against severe disease.”

“Down the line, probably, we would need to think of [offering booster doses]. But really, we don’t have strong arguments for it right now,” he says.

The developers of widely authorized COVID-19 vaccines have also kept maintaining that their vaccines are able to offer adequate protection against the development of severe disease.

Speaking to MNT, a Pfizer spokesperson emphasized:

“[I]t’s worth noting that we remain confident in the protection offered by the two-dose Pfizer/BioNTech COVID-19 vaccine (BNT162b2), which continues to show high efficacy against preventing severe disease and hospitalizations.”

“However,” they added, “we continue to take steps in parallel and follow the science, as we have done since the very start of this pandemic. By taking steps in parallel and remaining vigilant, we believe we can remain one step ahead of this virus.”

In a comment provided to Nature, Prof. Robert Aldridge, an infectious disease epidemiologist from University College London in the U.K., offers some cautionary thoughts. He says that whether or not national authorities decide to authorize additional booster doses in the near future will be, by necessity, “a difficult call, and it will almost certainly have to be made on incomplete evidence,” as researchers continue to weigh the real benefits of investigational boosters.

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