Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households

Background The effect of vaccination for COVID-19 on onward transmission is unknown. Methods A national record linkage study determined documented COVID-19 cases and hospitalisations in unvaccinated household members of vaccinated and unvaccinated healthcare workers from 8 th December 2020 to 3 rd March 2021. The primary endpoint was COVID-19 14 days following the first dose. Results The cohort comprised of 194,362 household members (mean age 31·1 ± 20·9 years) and 144,525 healthcare workers (mean age 44·4 ± 11·4 years). 113,253 (78·3%) of healthcare workers received at least one dose of the BNT162b2 mRNA or ChAdOx1 nCoV-19 vaccine and 36,227 (25·1%) received a second dose. There were 3,123 and 4,343 documented COVID-19 cases and 175 and 177 COVID-19 hospitalisations in household members of healthcare workers and healthcare workers respectively. Household members of vaccinated healthcare workers had a lower risk of COVID-19 case compared to household members of unvaccinated healthcare worker (rate per 100 person-years 9·40 versus 5·93; HR 0·70, 95% confidence interval [CI] 0·63 to 0·78). The effect size for COVID-19 hospitalisation was similar, with the confidence interval crossing the null (HR 0·77 [95% CI 0·53 to 1·10]). The rate per 100 person years was lower in vaccinated compared to unvaccinated healthcare workers for documented (20·13 versus 8·51; HR 0·45 [95% CI 0·42 to 0·49]) and hospitalized COVID-19 (0·97 versus 0·14; HR 0·16 [95% CI 0·09 to 0·27]). Compared to the period before the first dose, the risk of documented COVID-19 case was lower at ≥ 14 days after the second dose for household members (HR 0·46 [95% CI 0·30to 0·70]) and healthcare workers (HR 0·08 [95% CI 0·04 to 0·17]). Interpretation Vaccination of health care workers was associated with a substantial reduction in COVID-19 cases in household contacts consistent with an effect of vaccination on transmission.