Ring vaccination was found to decrease the risk for COVID-19 infection in areas with increased transmission of the SARS-CoV-2 Alpha variant, indicting its potential use as an adjunct to mass immunization. These findings were published in JAMA Network Open.

Researchers conducted a quasi-experimental, nonrandomized control study to evaluate the effect of ring vaccination in protecting against transmission of the SARS-CoV-2 Alpha variant among individuals in Montreal, Canada. The study included parents and teachers of children from 32 schools and 48 childcare centers in neighborhoods with a significantly increased risk for Alpha variant transmission. Participants were invited to receive 1 dose of a COVID-19 mRNA vaccine at 1 of 4 mass immunization sites between March and April 2021. The researchers assessed local vaccination rates to assign participants to 1 of 3 groups. The 3 groups comprised a primary intervention group (vaccination rate, ≥500 per 10,000 persons), a secondary intervention group (95-499 per 10,000 persons), and a control group (0-50 per 10,000 persons). Group comparisons were made prior to the intervention (December 1, 2020-March 16, 2021), during and immediately after the intervention (March 17-April 17, 2021), and 3 weeks after the intervention midpoint (April 18-July 18, 2021).

A total of 11,794 participants in Montreal received 1 dose of a COVID-19 mRNA vaccine. Of these participants, the mean age was 43 (range, 16-93) years, 5766 (48.9%) were included in the primary intervention group, and 9784 (83.0%) were parents. Among participants in the primary intervention group, the daily risk for COVID-19 infection was significantly increased compared with those in the control group, both before (unadjusted risk ratio [RR], 1.58; 95% CI, 1.52-1.65) and during (RR, 1.63; 95% CI, 1.52-1.76) the intervention period. In the weeks following receipt of COVID-19 ring vaccination, the risk for COVID-19 infection significantly decreased among only participants in the primary intervention group and the risk was no longer significantly different vs those in the other 2 groups (RR, 1.03; 95% CI, 0.94-1.12). The researchers then restricted the analysis to SARS-CoV-2 variants of concern (VOC) and found that the risk for infection was significantly decreased among participants in the secondary intervention group during both the intervention (RR, 1.27; 95% CI, 1.17-1.39) and postintervention periods (RR, 0.88; 95% CI, 0.82-0.94). In both the preintervention and intervention periods, the risk for infection with a SARS-CoV-2 VOC was marginally increased among participants aged 30 to 59 years compared with the overall group (RR, 1.79; 95% CI, 1.60-2.00).

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Study limitations included the nonrandomized design that potentially contributed to the decreased risk for COVID-19 infection observed among participants in the primary intervention group. In addition, differences between participants in the 3 groups in regard to COVID-19 testing and behavior modifications (eg, masking and social distancing) may have confounded the results. There also may have been residual confounding at the individual level because only population- and group-level data were included in the analysis.

According to the researchers, “…planners of COVID-19 immunization strategies should strongly consider adding ring vaccination as an adjunct to mass immunization in order to control transmission in specific areas based on local epidemiology.” They also suggested, “… after persons at [increased] risk for hospitalization and death are immunized, consideration should be given to targeting immunization to persons residing, working, or attending school in areas with [increased] or growing COVID-19 case counts, with the goal of stopping or slowing transmission.”


Périnet S, Cadieux G, Mercure SA, Drouin M, Allard R. Analysis of COVID-19 risk following a ring vaccination intervention to address SARS-CoV-2 alpha variant transmission in Montreal, Canada. JAMA Netw Open. 2022;5(2):e2147042. doi:10.1001/jamanetworkopen.2021.47042