In interpandemic periods, there are an estimated 1 billion cases of flu, 3–5 million cases of severe illness, and 300 000-500 000 deaths, each year, worldwide[7]. A number of studies have shown that a large proportion of influenza pneumonia in fact results from a synergy between the influenza virus and S. pneumoniae[5]’ [8]. The influenza virus increases colonization of the respiratory tract by Streptococcus pneumonia which then facilitates its invasion and the development of pneumonia. This may also occur for other bacteria such as H. influenza or Staphylococcus aureus. Influenza A infections can also have a significant influence on the risk of subsequent meningococcal meningitis[9, 10],[11, 12],[13] with variable strength for different influenza subtypes.
Here we observed significant temporal relation between influenza epidemics and the number of cases of sepsis hospitalized in the reference hospital of French Guiana. Despite being close to the equator, influenza seasonality was marked, presumably because of the close link with mainland France where influenza epidemics often affect air travelers to French Guiana who then propagate the epidemic locally. Although the population of French Guiana is small, the output ARIMA model estimates approximately one additional case of sepsis out of every 1000 cases of influenza which during an epidemic is not negligible. Therefore vaccination against seasonal influenza could have non negligible benefits in reducing the number of cases of sepsis. The vaccine for the northern hemisphere is effective in French Guiana, however the timing and the shape of the epidemic are different from what is observed in mainland France.[14] On average influenza epidemics in French Guiana last 22 weeks whereas in mainland France they last 9 weeks. The onset of the epidemic usually occurs 4 weeks after the beginning of the epidemic in mainland France. However, vaccine reimbursement policy only extends until December 31 of a given year. This is likely to limit the number of persons vaccinated and given the 1 sepsis per 1000 influenza cases observed here, extending reimbursement of the vaccine after December may allow benefits on sepsis.[14]
For Dengue, the initial perception from the bedside was that some patients developed other infections after dengue. This hypothesis was not vindicated by the available data. Therefore, post dengue “immunosuppression” does not seem to have any consequences on subsequent sepsis. Post dengue immune suppression affecting lymphocytes rather than neutrophils, the susceptibility to bacterial sepsis may not have been affected by a prior dengue episode.
Limitations
The limitations of the present study were possibly the insufficient time span and low patient numbers given the small population of French Guiana which could have masked less obvious interactions. In addition, the sepsis definition was broad, notably because of the small patient numbers when looking at specific diagnoses. The influenza and dengue cases were clinical suspicions which may not have been always accurate. However, they originated from a sentinel network working in close relation to the Health authorities and French Guiana’s Pasteur Institutes National Reference Laboratory for arboviroses and influenza. The predicted positive value of the criteria for defining suspected cases by the sentinel network in a proven epidemic context is high, therefore we are confident that the time series reflected the evolution of the studied epidemics.
In conclusion, contrarily to our hypothesis there was no link between dengue epidemics and sepsis. As observed elsewhere there was a significant link between flu epidemics and sepsis. The flu epidemics were highly seasonal and it was estimated that for every 1000 cases of flu there was one additional case of sepsis. In this tropical setting, influenza was highly seasonal, and improved vaccination coverage could have benefits on sepsis.