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 hospitalizations in unvaccinated household members of vaccinated and unvaccinated healthcare workers from 8th December 2020 to 3rd 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 years) and 144,525 healthcare workers (mean age 44.4 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. 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] 0.63-0.78). The effect size for COVID-19 hospitalization was similar, with the confidence interval crossing the null (HR 0.77 [0.53-1.10]). 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 [0.42-0.49]) and hospitalized COVID-19 (0.97 versus 0.14; HR 0.16 [0.09-0.27]). 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 [0.30-0.70]) and healthcare workers (HR 0.08 [0.04-0.17]). Conclusion: 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.