Although the majority of the French population is fully vaccinated, the circulation of the virus is still active in the country. In a context of lifting health restrictions, the fear of a resurgence of the epidemic and the appearance of new, more contagious variants is leading scientists to think of new strategies to continue promoting vaccination among populations that remain reluctant. A new modeling study conducted by researchers from Inserm and Sorbonne University at the Pierre Louis Institute of Epidemiology and Public Health shows that a so-called “reactive” vaccination strategy, in which vaccination is offered to everyone around an infected person, could have beneficial effects, reducing the number of Covid-19 cases in certain epidemic situations. The results of this work are published in the journal nature communications.
Mass vaccination campaigns against Covid-19 in many countries have considerably mitigated the pandemic. However, the rate of people vaccinated is now stagnating in Europe and the United States, due to logistical limitations and hesitancy on the part of the population to get vaccinated.
In March 2022, 79% of French people were fully vaccinated with a two-dose vaccination schedule and 53% had received the third booster dose. Although these figures are high, efforts to counteract the epidemic must be maintained because in a context of still high viral circulation and the lifting of health restrictions, the resumption of the epidemic and, with it, the appearance of more severe variants is not yet ruled out. contagious. .
In this context, many scientists believe there is a need to test other vaccine strategies that promote accessibility and acceptability for better efficacy.
Researchers from Inserm and the Sorbonne University have thus become interested in a so-called “reactive” vaccination strategy, which consists of vaccinating all the people who are around the cases at home and in the workplace or school. . This approach is already used in other epidemic contexts, for example in the face of waves of meningitis. In the context of Covid-19, it was occasionally used in the field in France, for example in Strasbourg within the Haute Ecole des Arts du Rhin (HEAR), following the discovery of a group of the delta variant.
What is “ring vaccination”?
In other epidemic contexts, for example, during certain Ebola outbreaks, other innovative strategies have been deployed to reach as many people as possible. The best known is the ring vaccination strategy, which consists of vaccinating the direct contact cases and the contact cases of the latter around a proven case.
The research team wanted to assess the effects of this reactive approach on viral circulation and the number of Covid-19 cases, in different epidemic scenarios. To build their model, the scientists relied on INSEE data to model a typical population, with the sociodemographic characteristics, social contacts, and professional situations of a population in a medium-sized French city.
Several parameters have also been integrated into the model, in particular the characteristics of the disease, vaccination coverage, the effectiveness of vaccines, restrictions on contacts in the workplace or in the community, their movements, or even the implementation strategies for tracing. contact cases.
Scientists were then able to study the impact of a reactive vaccination strategy in the context of various scenarios of epidemic dynamics. Thus, they show that in most 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 around 45% and viral circulation is high, the reduction in the number of cases in a two-month period ranges from 10 to 16%, if we compare a mass vaccination program with a program where reactive vaccination is implemented in parallel with mass vaccination.
The results suggest that this strategy is most effective when vaccination coverage is low and when combined with robust case and contact tracing measures.
When vaccination coverage is high, a reactive strategy is less interesting, since most of the infected person’s environment is already vaccinated. However, such an approach would still have the benefit of reaching people who have not yet been vaccinated and more easily convincing them of the usefulness of the vaccine. In fact, the fact of having been exposed to the virus increases the perception of risk and tends to make vaccination more acceptable.
“The model we have built allows us to consider reactive vaccination as an effective strategy to increase vaccination coverage and reduce the number of cases in certain epidemic scenarios, especially when combined with other measures such as effective tracking of contact cases. This is a tool that could also be reused and adapted in France in case another variant emerges and where the effectiveness of a reactive strategy to administer possible booster doses needs to be tested. This modeling may also be of interest for other countries with sociodemographic characteristics similar to France, in which vaccination coverage is lower”, explains Chiara Poletto, researcher at Inserm and last author of the study.