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Mixing AztraZeneca and Pfizer COVID-19 vaccines produces strong immune response: Study

The yet-to-be-published study found that both mixed schedules -- Pfizer followed by AstraZeneca, and vice versa -- induced high concentrations of antibodies against the SARS-CoV2 spike protein when doses were administered four weeks apart.

June 30, 2021 / 12:31 PM IST

Alternating doses of the Oxford-AstraZeneca and Pfizer-BioNTech vaccines generate robust immune response against COVID-19, according to a study led by researchers from the University of Oxford in the UK.

The yet-to-be-published study found that both mixed schedules -- Pfizer followed by AstraZeneca, and vice versa -- induced high concentrations of antibodies against the SARS-CoV2 spike protein when doses were administered four weeks apart.

The spike protein helps the virus to infect and enter the human cells, and current vaccines are targeted against it.

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The findings, posted on The Lancet pre-print server on June 25, suggest all possible vaccination schedules involving the AstraZeneca vaccine, known as Covishield in India, and Pfizer preventive could potentially be used against COVID-19.

COVID-19 Vaccine

Frequently Asked Questions

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How does a vaccine work?

A vaccine works by mimicking a natural infection. A vaccine not only induces immune response to protect people from any future COVID-19 infection, but also helps quickly build herd immunity to put an end to the pandemic. Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease, making the spread of disease from person to person unlikely. The good news is that SARS-CoV-2 virus has been fairly stable, which increases the viability of a vaccine.

How many types of vaccines are there?

There are broadly four types of vaccine — one, a vaccine based on the whole virus (this could be either inactivated, or an attenuated [weakened] virus vaccine); two, a non-replicating viral vector vaccine that uses a benign virus as vector that carries the antigen of SARS-CoV; three, nucleic-acid vaccines that have genetic material like DNA and RNA of antigens like spike protein given to a person, helping human cells decode genetic material and produce the vaccine; and four, protein subunit vaccine wherein the recombinant proteins of SARS-COV-2 along with an adjuvant (booster) is given as a vaccine.

What does it take to develop a vaccine of this kind?

Vaccine development is a long, complex process. Unlike drugs that are given to people with a diseased, vaccines are given to healthy people and also vulnerable sections such as children, pregnant women and the elderly. So rigorous tests are compulsory. History says that the fastest time it took to develop a vaccine is five years, but it usually takes double or sometimes triple that time.

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The study evaluated mix and match combinations of the two vaccines to see to what extent these preventives can be used interchangeably, potentially allowing flexibility in global vaccine roll-out.

The researchers enrolled 830 participants, including 463 who were given a second dose at an interval of 28 days.

The mean age of the participants was 57.8 years, 45.8 per cent were female, and 25.3 per cent were from ethnic minorities.

"The results show that when given at a four-week interval both mixed schedules induce an immune response that is above the threshold set by the standard schedule of the AstraZeneca vaccine," said Professor Matthew Snape, Associate Professor in Paediatrics and Vaccinology at the University of Oxford.

The researchers noted that an AstraZeneca/Pfizer schedule induced higher antibodies and T-cell responses than a Pfizer dose followed by AstraZeneca.

Both of these induced higher antibodies than the licensed, and highly effective ''standard'' two-dose AstraZeneca schedule, they said.

The highest antibody response was seen after the two-dose Pfizer-BioNTech schedule, and the highest T cell response from Oxford-AstraZeneca followed by Pfizer-BioNTech, according to the study.

The T cell response is an essential part of the host immune response to acute virus infection.

"These results are an invaluable guide to the use of mixed dose schedules, however the interval of four weeks studied here is shorter than the eight to 12-week schedule most commonly used for the Oxford-AstraZeneca vaccine," Snape said.

"This longer interval is known to result in a better immune response, and the results for a 12-week interval will be available shortly," he added.

Professor Jonathan Van-Tam noted that the data is a vital step forward, showing a mixed schedule gives people protective immunity against COVID-19 after four weeks.

"Our non-mixed vaccination programme has already saved tens of thousands of lives across the UK but we now know mixing doses could provide us with even greater flexibility for a booster programme," Van-Tam said.

He added that mixing doses of the two vaccines could support countries who have further to go with their vaccine rollouts and may be experiencing supply difficulties.

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first published: Jun 30, 2021 12:31 pm