Mix-Match Of AstraZeneca, Pfizer Doses Approved: How Does It Work?

Virologist T Jacob John told BOOM that a vaccine mixture of Covishield and Covaxin could be possible once clinical trials prove its efficacy

A group of Spanish scientists from Carlos III Health Institute found that people who received the first dose of AstraZeneca vaccine and followed it up with a dose of the Pfizer vaccine had more antibodies against SARS-CoV-2 when compared to the antibodies created after taking two doses of AstraZeneca.

This trial called the CombivacS- Clinical trial designed to evaluate the safety and immunogenicity of a dose of Comirnaty vaccine (Pfizer) in people who have previously received a dose of Vaxcevria vaccine (AstraZeneca) had 663 participants under the age of 60. These participants had received a shot of the AstraZenenca vaccine and were participating only after 8 weeks of receiving the first shot. The trials were conducted at two hospitals in Madrid and Barcelona, and one hospital in Vizcaya in Spain. Around 440 participants received the Pfizer vaccine as the second dose while 220 were a part of the control group.

This study showed that the antibody titers- presence, number, and level of antibodies in the body- against the SARS-CoV-2 in the vaccine combination was higher than what could be achieved with two doses of the adenoviral vector Oxford-AstraZeneca vaccine. While the proven efficacy of the Oxford-AstraZeneca vaccine lies close to 78%, the efficacy of the Pfizer-BioNTech vaccine is close to 95%, thus explaining why it boosts immunity. However, the researchers could not compare this antibody titer to the one produced when a person takes two doses of the Pfizer vaccine.

Even UK is trying out the similar combination of vaccines and the results have shown that the combination provides safety. Detailed results of the efficacy and the number of antibodies that are produced are expected in the upcoming weeks.

How Does Mixing of Vaccines Work?

All the research till date always suggested that a person taking one dose of a particular vaccine should take the second dose of the same vaccine as the technologies used to manufacture the vaccines are different.

This Spanish study is one of the first studies to find that a vaccine combination would actually boost immunity and help create more antibodies against SARS-CoV-2.

BOOM spoke to a virologist, Dr. T Jacob John to understand the science behind mixing the two vaccines and the risk of cross-reaction.

"The AstraZeneca vaccine acts as the primary dose for building immunity and the Pfizer vaccine provides the necessary booster to the primary dose." Dr. John stated.

Furthermore, he stated that even though they were differing technologies, both the vaccines attacked and acted upon the Spike protein of SARS-CoV-2.

"The AstraZeneca vaccine has genetic material- DNA coded for the spike protein which is carried through the adenoviral vector and enters the nucleus of cells and is transcribed to create messenger RNA that later is recognised by the body and the body starts eliciting an immune response." Dr.John continued. Transcription is the genetic process wherein DNA is copied to produce RNA which is further used to synthesise proteins through a process called translation.

Explaining the working of the Pfizer vaccine, Dr. John explains, "The Pfizer vaccine also tackles the spike protein and uses messenger RNA. As both are acting upon the same specified area, the different vaccine technologies ultimately deliver the same response. Since antibodies have increased, it is a good clinical trial to understand how two heterologous shots can act against SARS-CoV-2."

Can A Vaccine Combo Be Rolled Out In India?

India is currently administering the Oxford- AstraZeneca vaccine and indigenously produced Covaxin to citizens above the age of 18 in the country. In each of its guidelines, the Ministry of Health has stated that whichever dose the person has taken first should be repeated again for the second dose.

Covaxin's second dose is supposed to be taken after a period of four weeks while AstraZeneca's Covishield is now being recommended to be taken after a period of 12-16 weeks.

When asked if India can try such a combination of vaccines, Dr. John stated that there is not enough data supporting the combination as neither the government nor any laboratories have undertaken this study. He, however, added that there could be a possibility of an increase in the number of antibodies produced if this different dosing method is tried in India.

"In the country, one thing that we have to take precautions about is that Covaxin is a fully inactivated virus carrying genes for other areas of the vaccine including the Spike protein. There could be chances of cross-reaction if the a Covishield dose is followed by Covaxin. We need to initiate clinical trials immediately to gauge whether mixing of vaccines can be implemented in the country," explained Dr. John. Cross- reactivity occurs when a different chemical or biological reaction apart from the expected reaction occurs in the body. In case of the Covishield- Covaxin mixing, the cells of the vaccine recipient could react differently with the genes of the inactivated killed SARS-CoV-2 virus present in Covaxin.

Also Read:Door to Door Vaccination If Centre Frames Policy: BMC to Bombay HC


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