Impact of BNT162b2 vaccination and isolation on SARS-CoV-2 transmission in Israeli households: an observational study
Authors: Layan Maylis , Gilboa Mayan , Gonen Tal , Goldenfeld Miki , Meltzer Lilac , Andronico Alessio , Hozé Nathanaël , Cauchemez Simon , Regev-Yochay Gili 
Published: July 2021

Background

Massive vaccination rollouts against SARS-CoV-2 infections have facilitated the easing of control measures in countries like Israel. While several studies have characterized the effectiveness of vaccines against severe forms of COVID-19 or SARS-CoV-2 infection, estimates of their impact on transmissibility remain limited. Here, we evaluated the role of vaccination and isolation on SARS-CoV-2 transmission within Israeli households.

Methods

From December 2020 to April 2021, confirmed cases were identified among healthcare workers of the Sheba Medical Centre and their family members. Households were recruited and followed up with repeated PCR for a minimum of ten days after case confirmation. Symptoms and vaccination information were collected at the end of follow-up. We developed a data augmentation Bayesian framework to ascertain how age, isolation and BNT162b2 vaccination with more than 7 days after the 2nd dose impacted household transmission of SARS-CoV-2.

Findings

210 households with 215 index cases were enrolled. 269 out of 687 (39%) household contacts developed a SARS-CoV-2 infection. Of those, 170 (63%) developed symptoms. Children below 12 years old were less susceptible than adults/teenagers (Relative Risk RR=0·50, 95% Credible Interval CI 0·32-0·79). Vaccination reduced the risk of infection among adults/teenagers (RR=0·19, 95% CI 0·07-0·40). Isolation reduced the risk of infection of unvaccinated adult/teenager (RR=0·11, 95% CI 0·05-0·19) and child contacts (RR=0·16, 95% CI 0·07-0·31) compared to unvaccinated adults/teenagers that did not isolate. Infectivity was significantly reduced in vaccinated cases (RR=0·22, 95% CI 0·06-0·70).

Interpretation

Within households, vaccination reduces both the risk of infection and of transmission if infected. When contacts were not vaccinated, isolation also led to important reductions in the risk of transmission. Vaccinated contacts might reduce their risk of infection if they isolate, although this requires confirmation with additional data.

Funding

Sheba Medical Center.

Research in context

Evidence before this study

The efficacy of vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmissions in households remains understudied. On June 28, 2021, we searched PubMed and medRxiv for articles published between December 1, 2020, and June 28, 2021, using the following combination of search terms: (“COVID-19” OR “SARS-CoV-2”) AND (“household*” OR “famil*”) AND “transmission” AND “vaccination”. Our search yielded two articles that investigated the effect of vaccination on SARS-CoV-2 transmission in households. They showed a lower risk of infection in households with vaccinees. Vaccine efficacy on the risk of infection was estimated to 80% after the 2nd dose, and vaccine efficacy on the risk of transmission if infected was estimated to 49% 21 days after the 1st dose. However, these estimates are derived from surveillance data with no active follow-up of the households. In addition, the impact of isolation precautions has not been assessed.

Added value of this study

Based on the active follow-up of households of health care workers from the Sheba Medical Center in Israel, we estimated the effect of vaccination on household transmission. To our knowledge, our study is the first to conjointly investigate the effect of vaccination, age, and isolation precautions on the risk of infection and the risk of transmission in households while accounting for tertiary infections in the household, infections within the community, the reduced infectivity of asymptomatic cases, misidentification of index cases, and household size. Our study confirmed the high efficacy of BNT-162b2 vaccination to reduce infection risk and transmission risk. It also suggests that isolation might remain beneficial to vaccinated contacts.

Implications of all the available evidence

Vaccination reduces susceptibility to infection and case infectivity in households. Isolation precautions also mitigate the risk of infection and should be implemented whenever a household member is infected. They might remain beneficial to vaccinated contacts.