A study of 700 people who tested positive for influenza suggests that their risk of infecting household contacts was 18.8% and that the estimated effectiveness of flu vaccines against secondary infections is 21.0%.
Vanderbilt University–led researchers tested nasal swabs for evidence of influenza and analyzed symptom diaries for up to 7 days from 699 participants and their 1,581 household members during the 2017-18, 2018-19, and 2019-20 respiratory virus seasons. Case-patients had sought care at clinics in Tennessee and Wisconsin after experiencing flu-like symptoms.
"The degree to which vaccines reduce secondary infections after exposure to influenza, especially in high transmission settings, like households, may be especially informative to individuals at increased risk for severe influenza complications," the study authors wrote.
The findings were published last week in JAMA Network Open.
High risk for unvaccinated household members
The median age of participants with primary infections was 13 years, 54.5% were female, and 49.1% were vaccinated against flu. The median age in household contacts was 31 years, 52.7% were female, 50.1% were vaccinated, and 22.5% were diagnosed as having flu during follow-up. The median interval between index and secondary cases was 3 days.
Complementary preventive measures could include isolation of ill household members, improved ventilation, hand hygiene, disinfection of surfaces, use of masks and covering coughs and sneezes, and antiviral prophylaxis.
The secondary flu infection risk among household contacts was 18.8%. The secondary infection risk was highest among children younger than 5 years old (20.3% for influenza A and 15.9% for influenza B). Seven percent of secondary infections were asymptomatic.
The estimated vaccine effectiveness (VE) against secondary infection among unvaccinated household contacts was 21.0%. Estimated VE for preventing influenza B among household contacts was 56.4% overall, 88.4% among those aged 5 to 17 years, and 70.8% among those aged 18 to 49 years.
The estimated VE was 5.0% against influenza A and 56.4% against influenza B. The VE against H1N1 and H3N2—both "A" strains—was estimated as 21.4% and −26.9%, respectively.
"Our study showed that following introduction of influenza virus infections in households, there is a high risk of transmission to household members," the researchers wrote. "During the study period, influenza vaccination was associated with a reduced risk of laboratory-confirmed influenza virus infection, especially influenza B virus."
"Complementary preventive measures could include isolation of ill household members, improved ventilation, hand hygiene, disinfection of surfaces, use of masks and covering coughs and sneezes, and antiviral prophylaxis," they added.
