Indirect protection of children from SARS-CoV-2 infection through parental vaccination

Abstract

Children not vaccinated against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may still benefit from vaccines through protection from vaccinated contacts. We estimated the protection provided to children through parental vaccination with the BNT162b2 vaccine. We studied households without prior infection consisting of two parents and unvaccinated children, estimating the effect of parental vaccination on the risk of infection for unvaccinated children. We studied two periods separately-an early period (17 January 2021 to 28 March 2021; Alpha variant, two doses versus no vaccination) and a late period (11 July 2021 to 30 September 2021; Delta variant, booster dose versus two vaccine doses). We found that having a single vaccinated parent was associated with a 26.0 and a 20.8% decreased risk in the early and late periods, respectively, and having two vaccinated parents was associated with a 71.7 and a 58.1% decreased risk, respectively. Thus, parental vaccination confers substantial protection on unvaccinated children in the household.

Fig. 1.. Indirect effect of the BNT162b2 mRNA COVID-19 vaccine by age group and household size.

Indirect VE (one minus the incidence rate ratio) of one vaccinated parent and two vaccinated parents on the probability of infection of a susceptible child within the household, overall and within age group and household size categories. Points represent the point estimates, and error bars represent the 95% CIs. The top part shows the early study period (vaccination with two doses at least 7 days prior versus no vaccination; Alpha variant), and the bottom part shows the late study period (receipt of the booster dose versus no receipt of the booster dose; Delta variant). The numeric results included in this figure are presented in table S3.

Fig. 2.. Mechanism of disease transmission.

An illustration showing the indirect effect of parental vaccination on children’s risk of SARS-CoV-2 infection and two of its composite parts: the direct effect of vaccination on the parents (estimated as the incidence rate ratio of parental infection between vaccinated and unvaccinated parents) and the risk of transmission from an infected parent to his or her children (estimated as the odds ratio of an infected parent infecting at least one child in the household). We do not expect the indirect risk to equal the product of the direct risk and infectiousness because children may also be infected outside of the household or, potentially, through the other parent. Estimates shown are from the early study period, in which parents vaccinated with two vaccine doses at least 7 days prior were compared with unvaccinated parents and the dominant variant was Alpha.