Focus on flu vaccination

Vaccination was one of the topics discussed during the first day of the JNI. Among the work presented was the RESPIVAC study, funded by ANRS MIE, whose main objective is to assess the effectiveness of vaccination against viral respiratory infections in hospitalised adults.

Oussama Zidi, from Inserm US19 in Villejuif, presented the results of this national prospective study, which also involved Inserm teams from Saint-Etienne, Dijon and Paris; hospital-university teams (Saint-Etienne University Hospital, Rennes University Hospital, Hospices Civils de Lyon and Cochin Hospital in Paris); as well as teams from I-REIVAC, a network dedicated to clinical research in vaccinology, and F-CRIN, a national platform for developing French clinical research.

We asked Oussama Zidi a few questions to find out a little more.

• What is the context for your work?

The ANRS 0285s RESPIVAC study was conducted in eight university hospitals in the I-REIVAC network to specifically assess the effectiveness of vaccination in hospitalised patients with severe forms of influenza − adults admitted to hospital with severe acute respiratory syndrome.

Every winter in France, influenza is responsible for more than 20,000 hospital admissions and around 9,000 deaths, and the 2024–2025 season was particularly severe. To prevent severe disease, vaccination is recommended for older people and at-risk patients. Vaccine effectiveness varies from year to year because viruses mutate. In addition, people who have been vaccinated may have a weaker response to vaccination.

This is why it is essential to reassess the real-world effectiveness of this vaccination each season. In the RESPIVAC study, we used a protocol based on the so-called test-negative design.

• Can you tell us a little more about this method?

Today, this is the reference method for estimating real-world vaccine effectiveness. The principle is simple: all hospitalised patients were tested to determine whether or not they had the disease under study. At the time of recruitment, a reliable test was used to classify them as “positive” or “negative”.

Here, people hospitalised with severe acute respiratory syndrome underwent a nasopharyngeal swab. Patients who tested positive for influenza were considered the “cases”, while those who tested negative were the “controls”. The odds of vaccination were then compared in each group using an adjusted odds ratio, controlling for age, sex, comorbidities and month of inclusion, with stratification by centre. Vaccine effectiveness is expressed as: VE = 100 × (1 − adjusted OR).

The advantage of this approach is that it limits bias related to healthcare-seeking behaviour, because cases and controls come from the same population who sought care for the same symptoms.

• Could you present the results of your study?

Based on the 578 adults who were hospitalised for severe acute respiratory syndrome during the 2024–2025 season, vaccine effectiveness was estimated at 62% (95% CI: 41–75). It was particularly high among older people, who are a priority target group for vaccination. It rose to 79% (95% CI: 43–92) among those aged 65–75 years and reached 69% (95% CI: 40–84) among those over 75.

• What does this study tell us, and what will its implications be?

The study showed that the vaccine was effective in the population of adults hospitalised for severe acute respiratory syndrome. This reinforces the public health message in favour of vaccination coverage among at-risk groups. RESPIVAC’s contribution is to document this protection against severe forms of disease, complementing data from the French laboratories in the RELAB network.

The study will continue during the 2025–2026 and 2026–2027 seasons, enabling estimates of hospital-based vaccine effectiveness in France to be refined.