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.