PremiumNEW DELHI: At a time when clinical trials are being conducted on mixing and matching of doses of covid-19 vaccines, a World Health Organization (WHO) expert panel has warned that such studies require cautious interpretation.
WHO with support of the Strategic Advisory Group of Experts (SAGE) on Immunization and its covid-19 Vaccines Working Group is reviewing emerging evidence on use of heterologous priming schedules, also known as mix and match schedules.
In heterologous priming, the first and the second doses are different but based on the same vaccine platform. In heterologous boosting, vaccine doses are from different platforms.
In several studies, immune responses after a first dose of recombinant adenovirus vaccines such as Covishield followed by an mRNA vaccine such as that of Pfizer and Moderna have shown higher neutralising antibody levels and higher T cell-mediated immune responses in comparison with two doses of the recombinant vaccine and similar levels to those of two mRNA vaccines.
In India, the Indian Council of Medical Research (ICMR) citing its pre-print research, last week said, immunisation with a combination of first dose of Covishield followed by second dose of Covaxin was not only safe but also elicited better immunogenicity.
India's covid-19 working group of the National Technical Advisory Group on Immunisation (NTAGI) is considering allowing mix and match Covishield and Sputnik vaccines against covid-19, along with mixing of similar platform vaccines to be available in the country in due course.
"While these studies are encouraging, they require cautious interpretation given the limited sample sizes and lack of follow up, especially related to safety data, and the uncertain relevance of immunological readouts in relation to clinical impact," the WHO panel said in its interim statement on heterologous priming and not heterologous boosting.
India's drug regulator has given approval for a study to be conducted by the Christian Medical College (CMC), Vellore, on mixing of Covaxin and Covishield, official sources said on Wednesday. An expert panel of the Central Drugs Standard Control Organisation (CDSCO) on July 29 had recommended granting permission for conducting the study. The trial will involve 300 healthy volunteers.
The WHO expert panel is also reviewing emerging evidence on the need for and timing of an additional vaccine dose (booster dose 1) for the currently available covid-19 vaccines which have received Emergency Use Listing (EUL).
The panel in its interim guidance has recommended against a booster dose.
“In the context of ongoing global vaccine supply constraints, administration of booster doses will exacerbate inequities by driving up demand and consuming scarce supply while priority populations in some countries, or subnational settings, have not yet received a primary vaccination series. The focus for the time being remains on increasing global vaccination coverage with the primary series (either one or two doses for current EUL vaccines)," the panel said in its report.
"To date, the evidence remains limited and inconclusive on any widespread need for booster doses following a primary vaccination series," the panel said.
However, it said, introducing booster doses should be firmly evidence-driven and targeted at population groups in greatest need. The rationale for implementing booster doses should be guided by evidence on waning vaccine effectiveness, in particular a decline in protection against severe disease in general population or in high-risk people, or due to a circulating variant of concern.
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