The Clalit Research Institute, in collaboration with researchers from Harvard University and Tel-Aviv University, analyzed one of the world’s largest integrated health record databases to examine the indirect protection provided to unvaccinated children through parental vaccination with the BNT162b2 (Pfizer) vaccine.
Between June 2021 and October 2021, a wave of infections dominated by the Delta variant swept through Israel. During this period, the authors studied 181,307 unvaccinated children from 76,621 distinct households, and compared parents vaccinated with a third (booster) dose to parents who received only two doses at least five months prior. The study estimated that a single boosted parent decreased the risk of infection by 20.8% (95% confidence interval: 11.4% – 29.1%), while two boosted parents decreased the risk for infection by 58.1% (95% CI: 53.1% – 62.6%).
The authors also conducted a similar study during an earlier wave from December 2020 through March 2021, in which the Alpha variant was dominant. During this period, the authors studied 400,733 unvaccinated subjects (children and adolescents) from 155,305 distinct households. They compared unvaccinated parents to parents receiving two doses of the vaccine and found the indirect protection of parental vaccination on children to be even stronger than above.
Looking at specific subgroups, the authors found the indirect effects of parental vaccination to be fairly consistent across households of different sizes, and across different age groups of children, including the youngest age groups of 0-2 and 3-6 years old that are today still ineligible for vaccination.
The study further explores the mechanism of this effect, showing that not only is a vaccinated parent less likely to experience a documented infection, they are also less likely to transmit the infection to other household members if infected.
In summary, the results of this study show that parental vaccination confers substantial protection to children residing in the same household. These results reinforce the importance of increasing vaccine uptake among the vaccine-eligible population to curb the spread of the SARS-CoV-2 pandemic and protect those who cannot be vaccinated.
The research was conducted by Dr Samah Hayek, Dr. Galit Shaham, Yatir Ben-Shlomo, Dr. Eldad Kepten, Dr. Noa Dagan, Prof. Ran Balicer and Dr. Noam Barda from the Clalit Research Institute; Professor Marc Lipsitch of the Harvard T.H. Chan School of Public Health, and Prof. Ben Reis of Boston Children’s Hospital and Harvard Medical School; and Dr. Daniel Nevo from Tel-Aviv University.
Dr. Samah Hayek, senior researcher at Clalit Research Institute, said: “Vaccination not only provides direct protection, it also provides indirect protection to unvaccinated individuals living with the vaccinated individuals in the same household. This study highlights the indirect protection provided by vaccinated parents to their unvaccinated children, irrespective of household size or the child’s age, for both the Alpha and the Delta variants.”
Dr. Noam Barda, head of Epidemiology and Research at Clalit Research Institute and Lecturer at Ben-Gurion University, said: “While the age range for vaccination continually expands, many children and adolescents remain unvaccinated for different reasons. The current study shows that parental vaccination confers substantial protection for children living in the same household, emphasizing that vaccination not only protects vaccinated individuals, but also their loved ones.”
Prof. Ran Balicer, Director of the Clalit Research Institute and Chief Innovation Officer of Clalit, said: “This study of Clalit data suggests vaccinated and boosted parents provide impactful protection from Covid-19 to their unvaccinated children. By quantifying the indirect protection provided from vaccinated parents to unvaccinated children, several mechanisms of protection emerge, including lower transmission from vaccinated parents with a breakthrough infection to their unvaccinated children, as compared with unvaccinated parents.”
Prof. Marc Lipsitch, Director of the Center for Communicable Disease Dynamics and Professor at the Harvard T.H. Chan School of Public Health, said: “This is yet another example of the value of rich and complete data of the type maintained by Clalit to answer important scientific and public health questions. Knowing that vaccination protects not only them but their household should encourage any parent who is uncertain to get vaccinated and, when appropriate, boosted.”
Prof. Ben Reis, Director of the Predictive Medicine Group at the Boston Children’s Hospital Computational Health Informatics Program and Harvard Medical School, said: “Parents have a profound duty to protect their children from harm. Especially in families where children are too young to be vaccinated, this study provides stark evidence of the importance of parents being vaccinated – protecting not only themselves, but their children as well.”
Dr. Daniel Nevo, Senior Lecturer at the Department of Statistics and Operations Research at Tel Aviv University: “The results from this study exemplify how articulating causal questions and answering them using the right methods with rich data lead to valuable answers. We are now one step closer to understanding the impact of vaccines at scale.”
The research was funded in part by the Ivan and Francesca Berkowitz Family Living Laboratory Collaboration at Harvard Medical School and Clalit Research Institute.
Hayek S, Shaham G, Ben-Shlomo Y, Kepten E, Dagan N, Nevo D, Lipsitch M, Reis BY, Balicer RD, Barda N.
Indirect protection of children from SARS-CoV-2 infection through parental vaccination.
Science. 2022 Jan 27:eabm3087. doi: 10.1126/science.abm3087