Vaccine efficacy against severe COVID19, even for the delta variant, is so high that booster doses for the general population are not appropriate at this stage in the pandemic, according to a review by an international group of scientists published in The Lancet on Monday.
The review by experts, including those from the World Health Organisation (WHO) and the US Food and Drug Administration (FDA)summarised the currently available evidence from randomised controlled trials and observational studies published in peer-reviewed journals and pre-print servers.
The results reported from the observational studies, on average, show that vaccination had 95 per cent efficacy against severe disease both from the Delta variant and from the Alpha variant, and over 80 per cent efficacy at protecting against any infection from these variants.
The authors noted that across all vaccine types and variants, vaccine efficacy is greater against severe disease than against mild disease.
Although vaccines are less effective against asymptomatic disease or against transmission than against severe disease, even in populations with high vaccination coverage the unvaccinated minority are still the major drivers of transmission, as well as being themselves at the highest risk of serious disease, they said.
“Taken as a whole, the currently available studies do not provide credible evidence of substantially declining protection against severe disease, which is the primary goal of vaccination,” said lead author Ana-Maria Henao-Restrepo from WHO.
“The limited supply of these vaccines will save the most lives if made available to people who are at appreciable risk of serious disease and have not yet received any vaccine,” Henao-Restrepo said.
The authors noted that even if some gain can ultimately be obtained from boosting, it will not outweigh the benefits of providing initial protection to the unvaccinated.
If vaccines are deployed where they would do the most good, they could hasten the end of the pandemic by inhibiting further evolution of variants, they added.
The authors explained that even if levels of antibodies in vaccinated individuals wane over time, this does not necessarily predict reductions in the efficacy of vaccines against severe disease.
This could be because protection against severe disease is mediated not only by antibody responses, which might be relatively short lived for some vaccines, but also by memory responses and cell-mediated immunity, which are generally longer-lived, they said.
“The vaccines that are currently available are safe, effective, and save lives,” said study co-author Soumya Swaminathan, WHO Chief Scientist.
“Although the idea of further reducing the number of COVID-19 cases by enhancing immunity in vaccinated people is appealing, any decision to do so should be evidence-based and consider the benefits and risks for individuals and society,” Swaminathan added.
According to the review, if boosters are ultimately to be used, there will be a need to identify specific circumstances where the benefits outweigh the risks.
The authors also noted that the ability of vaccines to elicit an antibody response against current variants indicates that these variants have not yet evolved to the point at which they are likely to escape the memory immune response induced by the vaccines.
Even if new variants that can escape the current vaccines are going to evolve, they are most likely to do so from strains that have already become widely prevalent, they said.
Therefore, the authors said, the effectiveness of boosters developed specifically to match potential newer variants could be greater and longer lived than boosters using current vaccines.
A similar strategy is used for influenza vaccines, for which each annual vaccine is based on the most current data about circulating strains, increasing the likelihood that the vaccine will remain effective even if there is further strain evolution, they added.