Explained | What is a heterologous booster vaccine, and what are its advantages?

With COVID cases rising again, does a heterologous booster vaccine give the best shot at immunity? 

June 12, 2022 02:15 am | Updated June 21, 2022 06:13 pm IST

Studies observed that heterologous boosters show higher vaccine effectiveness than homologous boosters

Studies observed that heterologous boosters show higher vaccine effectiveness than homologous boosters | Photo Credit: Getty Images

Watch | What is a heterologous booster vaccine?

The story so far: With the number of COVID-19 cases creeping up the curve since May 24, a certain sense of guarded concern has naturally crept into pandemic control activities. With the number of cases, rising by the thousands on a daily basis, (on June 11, in a span of 24 hours, over 8,000 fresh cases were reported), repeat infections, breakthrough infections for those who have had the double dose of vaccine, the debate about getting a booster or precaution dose has gotten shriller.

What is the vaccine strategy?

Currently, the government supports vaccination through its healthcare network only for senior citizens, with or without co-morbidities, and certain categories including frontline workers and healthcare workers. It has given the private sector the go-ahead to vaccinate eligible groups with the precaution dose for a price. The debate has also increasingly called to question the appropriate vaccine to use in the precautionary dosage, whether it should be homologous, the same vaccine that was administered as primary dose, or if it should be heterologous — any vaccine other than the primary dose, for a better immune response.

Are heterologous booster shots better?

There is a growing body of scientific evidence to prove that when heterologous doses are used as booster shots, the immune response is more efficient.

In a May 23 article, in eBioMedicine, part of Lancet Discovery Science, Zoltán Bánki et al, basing their conclusions on a single-blinded, three-arm, partially randomised clinical trial, said: “This study clearly shows the immunogenicity and safety of heterologous AZ/BNT vaccination and encourages further studies on heterologous vaccination schedules.”

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In the March 17 issue of the New England Journal of Medicine , Robert L. Atmar et al discuss this point: “Heterologous prime-boost strategies may offer immunologic advantages to extend the breadth and longevity of protection provided by the currently available vaccines.” Demonstrating with the AstraZeneca base vaccine supplemented by an mRNA vaccine booster, the authors concluded that “the heterologous boost immunisation strategy provides an immune response that may prove to be beneficial for durable prevention and control of COVID-19.”

The arguments of an April 23 study conducted in Chile and published in Lancet Global Health by Alejandro Jara et al re-emphasised the pressing need to come to a conclusion about the most effective booster to be used: “Policy makers urgently need evidence of the effectiveness of additional vaccine doses and its clinical spectrum for individuals with complete primary immunisation schedules, particularly in countries where the primary schedule used inactivated SARS-CoV-2 vaccines.”

Many countries have restricted vaccine boosters to people at high risk of SARS-CoV-2 infection or related complications, including older adults, healthcare workers, and individuals with underlying health conditions, making it all the more important to prioritise the use of appropriate vaccines that would also enable a fair and equitable distribution of the doses.

The authors reported: “Our results suggest that a homologous or heterologous booster dose for individuals with a complete primary vaccination schedule with CoronaVac provides a high level of protection against COVID-19, including severe disease and death.” Without a doubt, in this study, heterologous boosters showed higher vaccine effectiveness than a homologous booster for all outcomes, providing additional support for a mix and match approach.

What are the other advantages?

In the NEJM paper, the authors pointed out that “an option to use heterologous booster vaccines could simplify the logistics of administering such vaccines, since the booster formulation could be administered regardless of the primary series.”

Since the first introduction of a vaccine for COVID-19 several more vaccines have entered the market. More options are now available across the spectrum, made from various vaccine candidates. While their availability in numbers might be low, the fact that there are more vaccines available in the market is likely to level the playing field.

What is the way ahead?

A comment on the Jara Lancet paper by Mine Durusu Tanriover and Murat Akova highlights how the results of the study provide insights to policy makers on how to manage the booster dose strategy after two doses of CoronaVac vaccination.

“It is now clear that, in a world where vaccine equity is a utopia, scientists can only strive for how to best use the available vaccines to reach for a maximum attainable benefit. A mix and match vaccination strategy, including inactivated vaccines for priming and heterologous boosters thereafter, seems to be a realistic policy.”

Editorial | Weighing the choices: on precautionary vaccine doses

It would be prudent for governments, including in India, to switch to a heterologous booster regimen to allow more persons eligible for the vaccine and able to pay for it access it in the market. As a community health move, it will also be easy for health administrators to deploy newer vaccines in the market as part of the public vaccination programme. Shortages of vaccines will not be an issue when the reliance on just a few vaccines disappears, public health experts said.

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