Editorial: Mixing vaccines

It is argued that such a mixed regimen will help overcome the challenges of the shortfall of certain vaccines and remove hesitancy, especially in settings where multiple Covid-19 vaccines are being used

Though mixing of vaccines may help give a fillip to the inoculation campaign, a cautious approach is needed till enough data is generated to confirm its efficacy and safety. A study by the Indian Council of Medical Research (ICMR) has shown that giving people a shot of Covishield, followed by Covaxin as the second dose, is safe and improves immunity. It is argued that such a mixed regimen will help overcome the challenges of the shortfall of certain vaccines and remove hesitancy, especially in settings where multiple Covid-19 vaccines are being used. However, an international level, multicentre randomised control trials need to be carried out to conclusively prove these findings. Worldwide, studies are now under way to understand if a combination of two different vaccines can outperform two doses of the same vaccine. Mixing should not be randomly done but be based on understanding multiple issues. In fact, the World Health Organisation (WHO) has cautioned against the practice of mixing vaccines since there is little data on record about its health impact. Europe’s drug regulator, too, has not made any definitive recommendation on combining shots. The ICMR study, though commendable, has severe limitations and is not enough to dispel doubts in people’s minds. The findings must be reconciled with scientific evidence available with the global health body. With research to establish the safety or otherwise of mixing vaccines continuing, it is not advisable to jump to conclusions. The government should take an informed decision on the matter once it has sufficient data to reassure the public.

Based on the basic principles of how vaccines work, the mix-and-match vaccine regimens are likely to work but there is a need for deeper studies to analyse the evidence in each of these vaccine combinations before any other recommendations can be made. If proved effective and safe, such heterologous strategies may be able to induce combined antibody and cell-mediated immune response resulting in stronger, broader, and long-lasting immunity. The potential advantages of such a strategy would result in flexibility in using vaccines based on fluctuation in supplies. Several countries, including Russia, Germany, South Korea and Thailand, are keen on switching to different vaccines for second doses or booster shots. The idea of heterologous boosting initially emerged in Europe when the AstraZeneca vaccine was found to be associated with blood clotting in a handful of individuals. A second dose with a different platform was recommended by many countries. In the case of India, despite administering over 50 crore doses so far, it has been beset with supply delays and vaccine hesitancy. As a result, the country is way off the target of covering the entire population by the end of this year.

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