The World Health Organization authorised COVID-19 vaccinations for usage in a "mix-and-match" schedule for the first two doses or as a booster on 16th December, validating a practice several countries have been pursuing for months out of necessity.

Mix-and-match

Countries that employ inactivated vaccines licenced by the agency for emergency use for first doses may switch to mRNA or vectored vaccines for future doses, according to a preliminary suggestion made by immunisation experts after their December 7 meeting. Countries that utilised vectored vaccinations initially might switch to mRNA vaccines for following doses, or vice versa.

As per WHO in its interim recommendation:

"Heterologous vaccination should be adopted only after careful evaluation of existing vaccine supplies, future vaccine supplies, and other access issues, as well as the possible advantages and dangers of the individual products being utilised."

Mixing strategies

WHO advocated for a 'flexible strategy' in terms of employing vaccinations with the same technology or combining them. Most of the studies that have been done so far show that using vectored or mRNA vaccines after inactivated vaccines, which is called heterologous use, causes more protective antibodies than using only inactivated vaccines, says the health body.

1. According to the WHO, mixing mRNA vaccines with vectored vaccines, regardless of dosage order, has consistently shown increased immunogenicity compared to employing the same technology referred to as homologous usage of vectored vaccinations.
2. Using mRNA vaccines mixed with vectored ones did not show clear evidence of better immunity than using mRNA vaccines alone.
3. Although some studies have found an increased incidence of side effects when vectored doses are combined with mRNA vaccinations, the majority of adverse effects are minor or moderate and often disappear within three days.
4. Three inactivated vaccines have been certified for emergency use listed by the agency: BIBP by Sinopharm, CoronaVac by Sinovac, both from China, and Covaxin by Bharat Biotech International, from India.
5. Inactivated vaccinations use conventional technology to destroy or "inactivate" SARS-CoV-2, the virus that causes COVID-19 before it is administered into the human body to develop protection.
6. Vectored vaccinations may now be made using a different virus, such as those that cause the common cold or are present in chimps. That virus has been engineered such that it does not cause disease but instead transmits genetic instructions to the human body, instructing it to create antibodies against SARS-CoV-2. AstraZeneca and Johnson & Johnson both utilise this technology in their COVID-19 vaccines, which have been authorised by the World Health Organization.
7. Pfizer-BioNTech and Moderna, both WHO-approved, utilise an mRNA technology platform including SARS-CoV-2 genetic material. Human cells are instructed to produce a harmless protein that is specific to the virus in order to teach the human body to build an immune response.

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

The author is a practising super specialist from New Delhi.