![]() ![]() 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. 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. Owing to the small sample size, the authors were not able to establish the vaccine effectiveness against asymptomatic infections versus symptomatic infections. Indeed, genomic and virological analysis confirmed only three index-contact pairs. Obviously, infection might also have occurred beyond the household level with unknown exposure in the community. 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). 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. But these results also highlight that breakthrough infections continue to occur in the vaccinated, with an attack rate of 25%. All breakthrough infections were mild, and no hospitalisations and deaths were observed. ![]() These results underpin the key message that vaccinated contacts are better protected than the unvaccinated. The SARs in household contacts exposed to the delta variant was 25% in vaccinated and 38% in unvaccinated contacts. 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. What is unique about this study is that both vaccinated and unvaccinated contacts were included, thereby allowing for stratified analyses by vaccination status, both for the index cases and the contacts. ![]() The study had two study arms, with the first group enrolling contacts only, and the second group enrolling both index and contact cases at a time when the delta variant was predominant. These participants contributed 8145 upper respiratory tract samples for up to 20 days, regardless of symptoms. 4Īnika Singanayagam and colleagues did a carefully designed cohort study whereby 602 community contacts (household and non-household) identified via the UK contact tracing system and 471 COVID-19 index cases were enrolled through the Assessment of Transmission and Contagiousness of COVID-19 in Contacts (ATACCC) study. 3 Data are lacking on whether the effect of vaccination on transmission is lower for the delta variant and new insights on this are provided by a study done in the UK when the delta variant was the predominant strain, reported in The Lancet Infectious Diseases. The now globally dominant delta variant is more transmissible 2 and associated with reduced vaccine effectiveness, particularly against mild breakthrough infections, whereas protection against severe disease is not greatly reduced. Before the emergence of the delta variant, it was reported that after at least one dose of the mRNA vaccine by Pfizer or the adenoviral vector vaccine by Astra Zeneca, the risk of symptomatic cases in household contacts of vaccinated cases was about 50% lower than that among household contacts of unvaccinated cases. What is less clear is whether vaccination not only directly protects individuals but reduces the risk of infection among the contacts of vaccinated people, particularly with respect to the now dominant delta variant. COVID-19 vaccines that have obtained WHO emergency use listing appear to have high efficacy against severe disease and death, but lower efficacy against non-severe infections, and emerging evidence suggests that protection against non-severe disease declines faster following vaccination than that against severe disease and death. ![]()
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