14 September 2021
In a recent Lancet Viewpoint opinion piece, a group of authors including NIHR Bristol Biomedical Research Centre’s (BRC) Jonathan Sterne and ARC West’s Julian Higgins emphasize that while the idea of further reducing the number of COVID-19 cases caused by the highly transmissible delta variant 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.
The authors summarise current evidence on vaccine efficacy, which does not appear to show a need for boosting in the general population, because efficacy of the existing vaccines against severe disease remains high. The authors argue that even if humoral immunity, measured by neutralising antibody titres, appears to wane, this does not necessarily predict reductions in vaccine efficacy over time. Additionally, reductions in vaccine efficacy against mild disease do not necessarily predict reductions in the (typically higher) efficacy against severe disease.
The authors suggest that 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. The ability of vaccines that present the antigens of earlier phases of the pandemic (rather than variant- specific antigens) to elicit humoral immune responses against currently circulating variants indicates that these variants have not yet evolved to the point at which they are likely to escape the memory immune responses induced by those vaccines.
The authors conclude:
“Currently available vaccines are safe, effective, and save lives. 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. 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. Indeed, WHO has called for a moratorium on boosting until the benefits of primary vaccination have been made available to more people around the world. This is a compelling issue, particularly as the currently available evidence does not show the need for widespread use of booster vaccination in populations that have received an effective primary vaccination regimen.”