To address the primary study outcome to establish the secondary attack rates (SARs) in household contacts, the vaccination statuses for 232 contacts exposed to 162 epidemiologically linked delta-variant-infected index cases were analysed. The SARs in household contacts exposed to the delta variant was 25% in vaccinated and 38% in unvaccinated contacts. These results underpin the key message that vaccinated contacts are better protected than the unvaccinated. All breakthrough infections were mild, and no hospitalisations and deaths were observed. But these results also highlight that breakthrough infections continue to occur in the vaccinated, with an attack rate of 25%. Time since vaccination in fully vaccination contacts was longer for those infected than those uninfected, suggesting that waning of protection might have occurred over time, although teasing out general waning versus reduced vaccine effectiveness due to delta is challenging owing to so many confounding factors.
SAR among household contacts exposed to fully vaccinated index cases (25%; 95% CI 15–35) was similar to household contacts exposed to unvaccinated index cases (23%; 15–31). Obviously, infection might also have occurred beyond the household level with unknown exposure in the community. Indeed, genomic and virological analysis confirmed only three index-contact pairs. Owing to the small sample size, the authors were not able to establish the vaccine effectiveness against asymptomatic infections versus symptomatic infections. This limitation together with the unconfirmed source of transmission in many of these index-contact pairs, suggests that the low SAR reported here should be interpreted with caution. Nevertheless, the findings raise concern that the effect of vaccination on reducing transmission might be lower for the delta variant compared with the variants that circulated in the UK before the emergence of delta.