Dr. Shyam P Lohani

THE current vaccination campaign is the largest in the history of mankind. At this time, more than 3.57 billion doses have been administered across 180 countries (Bloomberg, 2021). The latest rate of administration is roughly 30.2 million doses a day. Considering ideal vaccine equity, enough doses have now been administered to fully vaccinate 23.3 per cent of the world population. However, the distribution has been unequal. People from developed countries with the highest incomes are getting vaccinated more than 30 times faster than developing economies.
It has been estimated that vaccinating 70-85 per cent of the world population would enable a return to normalcy. At the current pace of 30.2 million a day, it would take approximately another year to achieve a high level of global immunity and about nine months to cover 75 per cent of the population globally. The vaccination rate is continuously increasing and new vaccines are in the pipeline. At this time, the United Arab Emirates (UAE) leads the world, with enough vaccinations to cover 75.7 per cent of its population (Bloomberg, 2021).
As of now, Nepal secured over 6.5 million doses of vaccines from two different manufacturers. Over 2.7 million doses of Covishield (AstraZeneca-Serum) and Vero Cell (Sinopharm) were administered to people as a first dose. More than 1.1 million people received both the doses and 1.6 million doses of one dose vaccine (full course) of Johnson and Johnson is going on (MoHP, 2021).

Evidences
With the COVID-19 vaccine rollout is well underway; some countries are starting to think about mixing vaccine products. In doing so, one vaccine is given as the first shot and the second vaccine of a different type by different manufacturer will be given several weeks later.
There are several reasons for mixing vaccine products. The supply of vaccines has been erratic and several countries are facing vaccine shortages. It is, therefore, helpful for those countries having vaccine supply shortages if two different vaccines can safely be given to the same individual. There is preliminary evidence that shows mixing two different vaccines leads to a potentially stronger immune response as compared to the two doses of the same vaccine. Few of the countries, however, are planning to use mixed vaccines due to the rare side effects of some vaccines.
If the mixing of two vaccines is proved safe and effective, it would help vaccination campaigns in almost all countries roll out uninterrupted as expected. For vaccines that require two jabs, public health officials in most of the countries typically recommend the second shot be the same as the first.
There is evidence that has shown the double-BioNTech, as well as the combined AstraZeneca-BioNTech vaccination, was more effective than the double-AstraZeneca alternative. Participants who had two BioNTech-Pfizer shots or a combination of AstraZeneca and BioNTech produced around 10 times more antibodies than those with two doses of AstraZeneca jabs. The same study showed neutralising antibodies slightly better than those achieved with two BioNTech shots if given four weeks apart. Therefore, the combination of AstraZeneca and BioNTech-Pfizer seems to be a favourable way to immunise people against COVID-19. The science behind this is not because the two vaccines are in any way similar, though. Those two vaccines represent two types of COVID vaccines currently available on the market.
A recent study in Spain (2021) found that people who received a first dose of the AstraZeneca vaccine followed by a booster with the Pfizer-BioNTech vaccine had better immune response than those getting two doses of the AstraZeneca vaccine.
The AstraZeneca vaccine is a traditional vector vaccine. It utilises a harmless virus to deliver instructions to human cells that help to build up antibodies against the coronavirus. The BioNTech vaccine is an mRNA vaccine that teaches human cells how to make a protein that in turn triggers an immune response and the production of antibodies. The Sinopharm product is an inactivated vaccine called Vero Cell. The storage requirements of Vero Cell make it highly suitable for low-resource countries. It is the first vaccine approved that carries a vaccine vial monitor in which a small sticker is attached that changes colour when the vaccine is exposed to heat allowing health workers to quickly notice whether the vaccine can safely be administered.

Mix-and-match
Although initial results of mix and match studies are encouraging, the World Health Organisation (WHO) has not yet approved its use. In Germany, people receiving two shots of the same vaccine as well as people with a mix-and-match vaccination are considered fully vaccinated. In Canada, mix-and-match vaccination is approved. In France, people who received a first dose of the AstraZeneca vaccine have been offered the vaccine made by Pfizer and BioNTech for their second dose.
Several studies are going on to look at the concept of mix-and-match. Although there is no scientific reason why mix-and-match is not feasible and safe but still we have to wait for further studies. The trial is ongoing looking for a combined efficacy of the AstraZeneca and the Sputnik V vaccines. As of today, there is no study underway to see the mix-and-match effect of Covishield and Vero Cell.
New strains have threatened renewed outbreaks. In the coming days, higher vaccination rates should contain the pandemic burden around the world. It takes around two weeks after a final vaccine dose for immunity to fully develop. After full vaccination, it is hoped that the use of masks and social distancing will no longer be necessary. Thus, it is urged that the vaccination campaign be resumed and continued till herd immunity is achieved.

(Prof. Lohani is the founder and academic director at Nobel College. lohanis@gmail.com)