United States: A study of 700 people with the flu found that they had an 18.8% chance of spreading the flu to others in their household. The study also showed that flu vaccines are about 21% effective at preventing these secondary infections. 

During the year 2017–2018, 2018–2019 and 2019–2020 respiratory virus seasons, using nasal swabs for viral testing and symptom diary data from 699 participants and their 1,581 household contacts, Vanderbilt University researchers. Case-patients had attended clinics in Tennessee and Wisconsin due to flu like illness. 

As reported by the Medicalxpress, “The extent to which vaccines prevent secondary cases after household exposure to influenza, particularly in settings with higher levels of transmission, may be more relevant and helpful to people who are at higher risk from severe influenza illnesses,” the study authors said. These findings were released last week in JAMA Network Open. 

The overall secondary flu infection risk of the household contacts was 18.8%. The risk of developing the secondary infection was found to be greatest in children less than 5 years of age (20.3% for Influenza A and 15.9% for Influenza B). 

Secondary infections were asymptomatic in 7%. The overall vaccine efficacy against secondary transmission was placed at 21.0% among unvaccinated contacts. Overall, estimated VE for preventing influenza B was 56.4%, 88.4% among persons aged between 5 and 17 years and 70.8% among persons aged between 18 and 49 years. 

The authors of the study mentioned that analyzing findings of influenza virus infections in households, higher chances of transmission to other members are likely to occur after the virus has entered a particular household. Specifically, a significant protective effect of influenza vaccination against laboratory confirmed influenza virus infection has been found during the period of the study was detected especially with the influenza B virus. 

“Other preventable measures may involve isolation of affected household members, improved household ventilation, strict hand washing, surface decontamination, facial masks and covering of the mouth and nose during sneezing and administration of antiviral drugs,” added the five.