The Strategic Advisory Group of Experts (SAGE) of the World Health Organization (WHO) has recommended using the same COVID-19 vaccine for both doses. SAGE made the recommendation in relation to the two-dose schedule of Astra Zeneca's ChAdOx1-S, which is sold as 'Covishield' in India and as 'Vaxzevria' in many other countries. The Indian government had recently approved a trial that involves mixing Covishield and Covaxin Covid-19 vaccines.
The group also said if different COVID-19 vaccines are inadvertently administered in the two doses, no additional doses of either vaccine are recommended at this time. The group updated its recommendations on the vaccine's usage on July 30.
Interestingly, recent studies had said immune responses after a first dose of the Astra Zeneca vaccine followed by an mRNA vaccine (Pfizer's BNT162b2 or Moderna's mRNA-1273) showed higher neutralising antibody levels and higher T cell mediated immune responses, in comparison with two doses of Astra Zeneca vaccine. Results from an observational study using ChAdOx1-S followed by mRNA-1273, also showed an increased but acceptable reactogenicity or common expected adverse reactions.
"While these studies are encouraging, they require cautious interpretation given the limited sample size and lack of follow up especially related to safety data. There are currently no vaccine effectiveness studies on the use of heterologous schedules. More observational data will be forthcoming and further recommendations will be issued," said the panel.
In the interim, it suggested countries can consider using ChAdOx1-S vaccine followed by a mRNA platform vaccine (i.e. BNT162b2 or mRNA-1273), only in particular situations of interrupted supply. There should be a minimum interval of 14 days between administration of ChAdOx1-S vaccine and any other vaccine against other conditions, it said.
WHO also recommended to use the vaccine in pregnant women only if the benefits of vaccination to the pregnant woman outweigh the potential risks. Some pregnant women are at increased risk of infection or have co-morbidities that add to their risk of severe disease. However, it recommended the use of the vaccine in breast feeding women.
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