In the opinion piece, the authors 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.

Read the full University of Bristol press release