COVID-19: ’Reactive’ Vaccination, Effective in Case of High Viral Circulation?

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Scientists are considering new strategies to continue to promote vaccination amo

Scientists are considering new strategies to continue to promote vaccination among the populations that remain hesitant © Mat Napo on Unsplash

Although the majority of its population is fully vaccinated, the virus continues to actively circulate in France. As health restrictions are being lifted, fears of a resurgence of the epidemic and of the emergence of new more contagious variants are leading scientists to consider new strategies to continue to promote vaccination among populations that remain hesitant. A new modeling study by researchers from Inserm and Sorbonne Université at the Pierre Louis Institute of Epidemiology and Public Health shows that a "reactive” vaccination strategy targeting homes, schools and workplaces where cases are detected could have beneficial effects, reducing the number of COVID-19 cases in certain epidemic situations. The findings of this research have been published .

Mass COVID-19 vaccination campaigns in many countries have greatly reduced the pandemic. However, the vaccination rate is now stalling in Europe and the USA due to logistical constraints and the vaccine hesitancy of part of the population.

In March 2022, 79% of French people were fully vaccinated with a two-dose regimen and 53% had received the third (booster) dose. While these figures are high, efforts to counter the epidemic must be maintained: against a background of ever-intense viral circulation and the lifting of health restrictions, a resurgence of the epidemic remains possible - and with it the appearance of more contagious variants.

In such a context, and to improve efficacy, many scientists therefore believe that other vaccine strategies promoting accessibility and acceptability should be tested.

Researchers from Inserm and Sorbonne Université were therefore interested in a "reactive” vaccination strategy, which involves vaccinating homes, schools and workplaces where cases have been detected. This approach is already used in other epidemics, for example against outbreaks of meningitis. In COVID-19, it has occasionally been used on the ground in France, for example in Strasbourg at the Haute Ecole des Arts du Rhine (HEAR), following the discovery of a cluster of the delta variant.

What is "ring vaccination”’

In other epidemic contexts, for example during some Ebola epidemics , other innovative strategies have been deployed to reach as many people as possible. The most well-known is that of ring vaccination, which involves immunizing contacts of confirmed cases or contacts of those contacts.

The research team wished to evaluate the effects of this reactive approach on viral circulation and the number of cases of COVID-19 in different epidemic scenarios. In order to build their model, the scientists used National Institute of Statistics and Economic Studies (INSEE) data to model a typical population with the sociodemographic characteristics, social contacts, and professional situations of a population the size of an average French city.
Several parameters were also incorporated into the model, such as disease characteristics, vaccination coverage, vaccine efficacy, restrictions on contact in workplaces or in the community, travel, and the implementation of contact tracing strategies.

The scientists were then able to study the impact of a reactive vaccination strategy on several scenarios of epidemic dynamics. They show that in the majority of the scenarios, with the same number of vaccine doses, a reactive strategy is more effective than other vaccination strategies in reducing the number of COVID-19 cases.

For example, in a context where vaccination coverage is approximately 45% and viral circulation is high, the reduction in the number of cases over a two-month period increases from 10 to 16% when comparing a mass vaccination program with a program in which reactive vaccination is set up in parallel to mass vaccination.

The findings suggest that this strategy is especially effective when vaccination coverage is low and when combined with robust contact tracing measures.

When vaccination coverage is high, a reactive strategy is less useful, as most of those in contact with an infected person are already vaccinated. Nevertheless, such an approach would still have the benefit of reaching people who are not vaccinated and convincing them more easily of the utility of the vaccine. Indeed, exposure to the virus increases one’s perception of the risks and tends to make vaccination more acceptable.

"The model we built enables reactive vaccination to be considered as an effective strategy for increasing vaccination coverage and reducing the number of cases in some epidemic scenarios, especially when combined with other measures such as effective contact tracing. This is a tool that can also be reused and adapted in France should another variant emerge and where the efficacy of a reactive strategy needs to be tested in order to administer any boosters. This modeling may also be of interest to other countries with sociodemographic characteristics similar to France, but lower vaccination coverage,” explains Chiara Poletto, Inserm researcher and last author of the study.

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