New Delhi: An international group of scientists, including those at the World Health Organization (WHO) and the US Food and Drug Administration (FDA), have in an expert review of Covid-19 vaccine boosters for the general population said it is not needed at this stage of the pandemic.
The review, published in The Lancet, says a booster dose is not needed at the moment because vaccine efficacy against severe Covid-19, even for the Delta variant, is quite high.
Currently available evidence on Covid-19 vaccine efficacy, from randomised controlled trials and observational studies published in peer-reviewed journals and pre-print servers, has been summarised in the review.
What are the findings of the observational studies?
Most observational studies have found that vaccines remain highly effective against severe Covid-19 disease, including the diseases occurring from the main variants of SARS-CoV-2.
The results reported from observational studies were averaged, and it was found that the efficacy of vaccination against severe disease occurring, both from the Delta variant and the Alpha variant, was 95 per cent, while the vaccine efficacy at protection against infection from these variants was 80 per cent. The observational studies also found the vaccine efficacy to be more against severe disease, than against mild disease, for all vaccine types and variants.
The effectiveness of vaccines against asymptotic disease or against transmission is less than the vaccine effectiveness against severe disease. Also, the unvaccinated minority in populations where most of the people are vaccinated, have been found to be the major drivers of transmission, and are at the highest risk of contracting severe Covid-19 disease.
What does the expert review say?
Dr Ana-Maria, the lead author of the review, says credible evidence of substantially declining protection against severe disease is not provided by current studies.
According to her, most lives will be saved if the limited supply of these vaccines is made available to the people who have not yet received a single dose and are at appreciable risk of severe disease. The benefits of initial protection provided to the unvaccinated individuals, by the first dose, will not be outweighed by the gains of boosters, she adds.
Dr Soumya Swaminathan, WHO Chief Scientist, says any decision to reduce the number of Covid-19 cases by enhancing immunity in vaccinated people, should be based on robust evidence and international scientific discussion, and must take into consideration the benefits and risks for individuals and society, according to the press release.
The fact that antibody levels in vaccinated individuals decrease over time does not necessarily predict reductions in the efficacy of vaccines against severe disease, the authors note. The authors explain that memory responses and cell-mediated immunity, which usually last for longer periods, are responsible for protection against severe disease, apart from antibody responses, which are comparatively short-lived. The authors also highlight the need to identify situations in which the risks are outweighed by the benefits if boosters are to be used.
The review also mentions that the number of vaccinated people will increase if the success in delivering vaccines to large populations is increased. The fact that variants have not yet evolved to the point where they are likely to escape the memory immune response induced by variants, could be inferred from the ability of vaccines to initiate an antibody response against the current variants, states the review.
Also, even if the variants evolve in a way such that they can escape the memory immune response of current vaccines, the mutation would probably occur from the strains which are widely prevalent already.
The review explains that boosters should be developed specifically to match these potential newer variants, as that would ensure greater and longer-lived effectiveness of the boosters, than the boosters using current vaccines. This will increase the likelihood of the vaccine remaining effective even after the strain evolves further. Each annual vaccine for influenza virus uses a similar strategy.
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