Vaccines provide disproportional protection to the increased hospitalisation risk posed by the Delta variant of SARS-CoV2: a meta-analysis

Variants of SARS-CoV2 that achieved global dominance (Alpha and Delta) have been associated with increased hospitalisation risk. A quantification of this risk across studies is currently lacking for Delta. Furthermore, how risk for severe disease changes in both vaccinated and unvaccinated individuals is important as the underlying risks determine public health impact. The surplus risk of Delta versus Alpha on hospitalisation was determined using random-effects meta-analysis. Infection with the Delta compared to the Alpha variant increased hospitalisation risk (unvaccinated: log HR 0.62, CI: 0.41 -- 0.84, P < 0.0001; linear HR 1.87). This finding should inform our response to future variants of concern, currently Omicron. SARS-CoV2 variants that achieve dominance, have achieved this through a higher rate of infection and this evolutionary trajectory has also come with a correlated higher risk of severe disease. The surplus risk posed by Delta was significantly lower however in the vaccinated (model estimate -0.40, CI: -0.73 -- -0.07, P = 0.017). Vaccination thus provided a disproportionate level of protection to hospitalisation with the Delta variant and provides further rationale for vaccination for SARS-CoV2 as a durable public health measure.