Can high COVID-19 vaccination rates in adults help protect unvaccinated children? Evidence from a unique mass vaccination campaign, Schwaz/Austria, March 2021

Abstract

Background After an outbreak of the SARS-CoV-2 Beta variant in the district of Schwaz/Austria, vaccination with Comirnaty vaccine (BNT162b2 mRNA, BioNTech-Pfizer) had been offered to all adult inhabitants (≥ 16 years) in March 2021. This made Schwaz one of the most vaccinated regions in Europe at that time (70% of the adult population took up the offer). In contrast, all other Austrian districts remained with low vaccine coverage. Aim We studied whether this rapid mass vaccination campaign provided indirect protection to unvaccinated individuals such as children (< 16 years) living in the same district. Methods To study the effect of the campaign we used two complementary approaches. We compared infection rates among the population of children (< 16 years) in Schwaz with (i) the child population from similar districts (using the synthetic control method), and (ii) with the child population from municipalities along the border of Schwaz not included in the campaign (using an event study approach). Results Before the campaign, we observed very similar infection spread across the cohort of children in Schwaz and the control regions. After the campaign, we found a significant reduction of new cases among children of −64.5% (95%-CI: −82.0 to −30.2%) relative to adjacent border municipalities (using the event study model). Employing the synthetic control method, we observed a significant reduction of −42.8% in the same cohort. Conclusion Our results constitute novel evidence of an indirect protection effect from a group of vaccinated individuals to an unvaccinated group.

Quasi-experimental methods for pharmacoepidemiology: difference-in-differences and synthetic control methods with case studies for vaccine evaluation

“…ATT estimands from quasi-experimental studies, however, are specific to the setting, exposure, outcomes, and exposed unit(s), as well as the contrast identified and other statistical analysis choices. Since the exposed units are rarely randomly selected (ie, the mass vaccination campaign in Schwaz occurred because of a prior outbreak 46 ), the ATT differs from the average treatment effect that is usually targeted by randomized trials and used in regulatory decisions. 56 It may be of specific interest in safety studies, as it represents the effect of removing the exposure from all exposed units (eg, compare Cowger et al 6 ).…”

“…They used both SCM analysis with other Austrian districts as controls and DiD analyses comparing Schwaz municipalities to neighboring municipalities outside the district (see Table 3 ). 46 All analyses reported strong indirect protection of children. 46 …”

“…After the authorization of COVID-19 vaccines to prevent symptomatic and severe illness in adults, questions remained about the effectiveness of the vaccines in providing indirect protection by preventing infection and transmission. 41 Winner et al 46 sought to assess the indirect protection of children via adult vaccination by comparing infection rates among Austrian children in the district of Schwaz—which had high adult vaccination uptake due to a campaign during a localized outbreak—to other districts with much lower rates as of March 2021. They used both SCM analysis with other Austrian districts as controls and DiD analyses comparing Schwaz municipalities to neighboring municipalities outside the district (see Table 3 ).…”

“…Negative controls included an in-space placebo test for SCM (ie, using the same method on nontargeted districts to create a null distribution) and assessment of preexposure parallel trends for DiD. 46 …”

Direct and indirect vaccination effects on SARS-CoV-2 infection in day-care centres: Evaluating the policy for early vaccination of day-care staff in Germany, 2021

“…indirect vaccination effect) by the following mechanisms: (i) reducing the risk of infection for vaccinated individuals and thus the contacts of unvaccinated individuals with infectious individuals and/or (ii) reducing the infectiousness of vaccinated individuals once they became infected. Previous studies provide evidence for indirect vaccination effects in the general population [ 2 , 3 ] and in households [ 4 - 6 ], but similar findings from the educational context or in other settings have not been published so far. Knowledge about indirect effects in daycares is of strategic importance when managing a pandemic or an outbreak as the number of people working in this setting is manageable, and day-care staff are easier to target with interventions than private households.…”