SARS-CoV-2 Secondary Attack Rates in Vaccinated and Unvaccinated Household Contacts during Replacement of Delta with Omicron Variant, Spain

Abstract

T he mass vaccination against SARS-CoV-2 that began at the end of 2020 reduced COVID-19-related mortality and severity in countries where substantial vaccine coverage was achieved (1,2). The vaccines also had a protective effect against the most recent variants (3,4). However, expectations that vaccines would stop community transmission of SARS-CoV-2 through herd immunity were quickly dampened by the early observation of infection and re-infection among vaccinated persons; waning vaccine effectiveness against transmission (VET) over time was observed (1,3) and confirmed in a large systematic literature review (5). Despite these results, protective effects of vaccination against infection among contacts have been reported (6). The vaccination status of index case-patients was also shown to play a role (6), underscoring the importance of vaccination for reducing the circulation of SARS-CoV-2. Nonetheless, the emergence of new variants of concern (VOC) with increased infectivity is an ongoing challenge for VET of currently licensed vaccines; early reports have shown a substantially lower VET for the Delta variant (B.1.617.2) compared with previous VOCs (7). Furthermore, rapid replacement of the Delta variant by Omicron (B.1.1.529) began in late 2021; the Omicron variant showed a transmission advantage because of its shorter generation time (S. Abbott et al., unpub. data, https://www.medrxiv.org/content/10.1101/2 022.01.08.22268920v1).Evaluating both variant virulence and SARS-CoV-2 VET under high vaccine coverage levels has major epidemiologic, social, and policy implications. We report the results of an observational study of household contacts of SARS-CoV-2-infected index case-patients during a Delta variant-dominant period from September to December 2021 and an Omicron variant-dominant period during January 2022 in a north-metropolitan area of Barcelona, Spain. We

Vaccinated COVID-19 Index Cases Are Less Likely to Transmit SARS-CoV-2 to Their Household Contacts: A Cohort Study

“…The high SAR observed in our study is consistent with the increase in rates observed in the pandemic over time and with the emergence of new variants. López-Muñoz et al [ 15 ], for their household contact study, estimated 58.2% SAR and 80.9% SAR for periods dominated by the Delta and Omicron variants, respectively. Madewell et al [ 14 ], in an update of their systematic review of household SAR, estimated 42.7% SAR (95% CI: 35.4%, 50.4%) for periods dominated by the Omicron variant.…”

“…While a direct comparison between variants is difficult, as vaccination levels and social restrictions varied, SAR estimates by Madewell et al [ 14 ] for the Omicron (42.7%), Alpha (36.4%), and Delta (29.7%) variants were higher than the overall SAR of 18.9% previously reported for the earlier pandemic phase when the wild-type variant was prevalent. Transmission levels have been reported to be higher in households than in other community settings due to exposure intensity and multiple opportunities for transmission [ 9 , 16 ] and due to reduced use of protective measures such as face masks by vaccinated household contacts [ 15 ]. Most protocols still recommend home confinement for people infected with SARS-CoV-2 to reduce transmission in community settings, so studies of household transmission and control measures will continue to be a priority [ 16 ].…”

Case clustering, contact stratification, and transmission heterogeneity of SARS-CoV-2 Omicron BA.5 variants in Urumqi, China: An observational study

“…As a measure of transmission probability per contact, SAR was always treated as an average characteristic across the whole case population or contact settings [ 24 , 38 , 39 ]. We fitted the contact tracing data to the beta-binomial distribution, assuming SAR to be a random variable following a beta distribution.…”

Environmental predictors of SARS-CoV-2 infection incidence in Catalonia (northwestern Mediterranean)

“…The limited duration of protective immunity against infection and the high genomic variability of the pathogen increase the rate of repeated infection 5,6 and extend the persistence of coronavirus over time 7 . This is especially evident with recent variants like Delta (B.1.617.2) and Omicron (B.1.1.529) and their sub-variants (BA.04 and BA.05) 8 . This new scenario may likely evolve towards an endemic disease, possibly controlled by weather conditions 9 that will cause outbreaks or seasonal peaks similar to most common respiratory infections 10 .…”