Vaccine effect on household transmission of omicron and delta SARS-CoV-2 variants in South Korea

ABSTRACT

Background. We aimed to evaluate household secondary attack rate (SAR) of SARS-CoV-2 variants, omicron and delta, based on index case and contact vaccination status, and antibody level effect of vaccinated index cases on household transmission. Methods. A prospective study of 92 index cases and 197 quarantined household contacts was performed. Tests for SARS-CoV-2 variant type and antibody level were conducted in index cases, and results of PCR tests (during the quarantine period) were collected from contacts. Association of antibody levels in vaccinated index cases and SAR was evaluated by multivariate regression analysis. To evaluate the association between antibody levels in vaccinated index cases and household transmission, the probability of transmission was calculated using the logistic regression model. We refit the model for the probability of transmission using the variables that could affect the household transmission (meal with index, contact with index case >=5/day, respiratory symptoms and immunocompromised status of index patients, and vaccination status of contacts). For this analysis, only vaccinated index cases who tested antibody levels within 7 days of symptom onset or diagnosis, whichever was earlier, were included. The link between index cases and household contacts according to their vaccination status is shown in Figure. A total of 40, 27, and 10 contacts were linked with 20 unvaccinated index cases, 14 booster-unvaccinated index cases, and 7 booster-vaccinated index cases with delta variant infection, respectively. Among the contacts of index cases with omicron variant infection, 20, 65, and 35 contacts were exposed to 7 unvaccinated, 25 booster-unvaccinated, and 9 booster-vaccinated index cases, respectively. Results. SAR was higher in households exposed to omicron variant than those exposed to delta variant. SAR was 35% and 23% for unvaccinated and vaccinated delta variant exposed contacts, respectively. SAR was 44% and 41% for unvaccinated and vaccinated omicron exposed contacts, respectively. Booster dose immunisation of contacts or vaccination of index cases reduced SAR of vaccinated omicron variant exposed contacts. In a model with adjustment, anti-receptor binding domain antibody levels in vaccinated index cases were inversely correlated with household transmission of both delta and omicron variants. Neutralising antibody levels had a similar relationship. When we performed sensitivity analyses using 3 day and 5 day thresholds for the period of antibody test, the results did not differ from the main analyses. In a model with adjustment for factors that could affect the household transmission, the estimated probabilities of SARS-CoV-2 infection among 132 contacts according to the level of anti-RBD antibodies (A) and neutralising antibodies (B) of 62 index cases. Serum samples for antibody tests were collected within 7 days of symptom onset or diagnosis, whichever was earlier, in vaccinated index cases. Shaded areas indicate 95% confidence intervals.