Vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) prevents infection and reduces the severity of coronavirus disease 2019 (Covid-19) in vaccinated people.
We investigated whether vaccination would reduce household transmission in the context of post-vaccination infection.
We analyzed data from the Household Assessment of Transmission (HOSTED) dataset, which has information on all laboratory-confirmed cases of Covid-19 in England and which links data from all people who share the same direction. We then linked it to data on all Covid-19 vaccinations in England.
We compared the risk of secondary infection (defined as a positive SARS-CoV-2 test between 2 and 14 days after the positive test for the index case) among unvaccinated household contacts of people with SARS-CoV-2 infection who had received at least one dose of the ChAdOx1 nCoV-19 (Oxford and AstraZeneca) or BNT162b2 (Pfizer-BioNTech) vaccine 21 days or more before testing positive with the risk among unvaccinated household contacts of unvaccinated infected people.
We fit logistic regression models adjusting for age and sex of the person with the Covid-19 index case (index patient) and household contact, geographic region, calendar week of the index case, deprivation (a composite score of socioeconomic factors). . and other factors), and the type and size of the household. We also considered the timing of effects among index patients who had been vaccinated at any time up to the date of the positive test.
Table 1
Number of household contacts and secondary cases of Covid-19, according to the vaccination status of the index patient and adjusted odds ratios.
Between January 4 and February 28, 2021, there were 960,765 household contacts of unvaccinated index patients and there were 96,898 secondary cases of Covid-19 (10.1%). The number of secondary cases according to the vaccination status of the index patient and the results of the logistic regression models are shown in Table 1.
Overall, the likelihood of household transmission was approximately 40% to 50% lower in households of index patients who had been vaccinated 21 days or more before testing positive than in households of unvaccinated index patients; The findings were similar for the two vaccines.
The majority of vaccinated index patients in our data set (93%) had received only the first dose of vaccine. Assessment of infection risks among household contacts according to the timing of the index patient’s vaccination showed protective effects when the vaccine had been administered at least 14 days before the positive test.
HOSTED does not include data on symptoms or cycle threshold values and only has information on diagnosed cases. Among index patients, those who had been vaccinated were likely to have less severe symptoms and may have been less infectious than those who were not vaccinated.
Studies involving active contact tracing and using serological testing have shown higher rates of household transmission than were observed in our study; Bias could occur if case ascertainment differs between household contacts of vaccinated people and those of unvaccinated people. Our findings regarding the timing of vaccination of index patients are consistent with previous data on the timing of individual protection after vaccination and therefore support the overall findings.
There may have been misclassification of index and secondary cases, which are determined based on testing dates; however, such misclassification would tend to attenuate the estimated protective effect of vaccination. Data are needed to report the reduction in virus transmissibility after receipt of two doses of vaccine. It will be important to consider these findings alongside other emerging evidence to inform the benefits of vaccination.
