evidence: However, as the delta variant became predominant, concerns arose that mRNA based vaccines might be less effective.1415 The higher transmissibility of delta led to a surge in infections, hospital admissions, and deaths in the US.16 These cases have occurred overwhelmingly among unvaccinated people but have also included breakthrough cases.171819 Although most studies have found sustained effectiveness of mRNA based vaccines against hospital admission with covid-19 during periods overlapping with or during the delta surge,202122 decreased vaccine effectiveness against infection with delta in some studies has been reported (for example, 51-75%).222324 Whether these findings are due to lower protection against delta, waning vaccine immunity over time, or other factors is unclear. [...] The findings of this study have implications for booster doses, which have been authorized in the US and other countries in certain populations. Our findings of reduced vaccine effectiveness of mRNA-1273 against infection with the delta variant, particularly in older people, suggest that booster doses are needed as part of covid-19 vaccination strategies. However, efforts to deploy booster doses must not replace efforts to reach unvaccinated people, who account for most hospital admissions and deaths with covid-19. Booster dose strategies must also prioritize global vaccine production and allocation.