In-ward transmission of influenza frequent, according to a single-center study

Researchers at a hospital in Sweden found influenza clusters distributed between December and May in 20 different wards, 70% of which were internal medicine or cardiology wards.


Influenza may be frequently transmitted between hospitalized adults, a retrospective study found.

Researchers identified hospitalized adults with laboratory-verified influenza A virus infection during the 2016-2017 influenza season at a large hospital in Sweden. They characterized cases according to age, sex, comorbidity, receipt of antiviral therapy, viral load expressed as cycle threshold values, length of stay, 30-day mortality, and whether the infection met criteria for a health care-associated influenza (influenza-like illness or acute respiratory infection with onset of symptoms 48 hours or more after hospital admission or 48 hours or less after discharge).

To detect clustering, respiratory samples positive for influenza A virus collected on the same wards within seven days were chosen for whole genome sequencing, and phylogenetic analysis was performed. Concurrent influenza A virus strains from patients with community-acquired infection were included for reference. Results were published online on Feb. 3 by Clinical Infectious Diseases.

Overall, 435 cases of influenza A virus infection were identified, and 114 (26%) met the criteria for health care-associated infection. Seventy-four (65%) of health care-associated infections possibly involved in-ward transmission. The overall 30-day mortality rate was higher in patients with health care-associated infection compared to those with infection that wasn't associated (9.6% vs. 4.6%), although the difference was not significant in multivariable analysis, where age was the only independent risk factor for death (P<0.05).

Whole genome sequencing was successful in 124 hospital cases, and 60 (48%) sequenced samples belonged to an in-ward cluster or pair. Analysis identified eight closely related clusters (involving three or more cases) and another 10 pairs of strains supporting in-ward transmission. Clusters were distributed between December and May, and a total of 20 different wards were affected, 14 (70%) of which were internal medicine or cardiology wards. In three cases, there was a close relationship between an in-ward cluster and a single strain from another ward (recent transfer from an affected ward in one case and adjacent localization of wards in the other two cases). A few reference cases from the community corresponded well in time to an in-ward cluster.

The study authors noted that all data were collected retrospectively, and no information was available regarding adherence to infection control measures. They added that they did not assess potential links between patients and health care workers.

“In conclusion, we found that in-ward transmission of [influenza A virus] occurs frequently and that health-care-associated influenza may have a severe outcome,” the authors wrote. “[Whole genome sequencing] may be used for outbreak investigations as well as for evaluation of the effect of preventive measures.”