Background: Understanding the attack rate of influenza infection and the proportion who become ill by risk group is key to implementing prevention measures. While population-based studies of anti-haemagglutinin antibody responses have been described previously, studies examining both anti-haemagglutinin and anti-neuraminidase antibodies are lacking.
Methods: In 2015, we conducted a sero-epidemiologic cohort study of individuals randomly selected from a population in New Zealand. We tested paired sera for haemagglutinin-inhibition (HAI) or neuraminidase-inhibition (NAI) titres for seroconversion . We followed participants weekly and performed influenza PCR for those reporting influenza-like illness (ILI).
Results: Influenza infection (either HAI or NAI seroconversion) was found in 321 (35%; 95%CI:32-38%) of 911 unvaccinated participants, of which 100 (31%) seroconverted to NAI alone. Young children and Pacific peoples experienced the highest influenza infection attack rates, but overall only a quarter of all infected reported influenza-PCR-confirmed ILI and one-quarter of these sought medical attention. Seroconversion to NAI alone was higher among children aged <5 years vs. those aged ≥5 years (14% vs 4%; p<0.001) and among those with influenza B vs A(H3N2) virus infections (7% vs 0.3%; p<0.001).
Conclusions: Measurement of anti-neuraminidase antibodies in addition to anti- hemagglutinin antibodies may be important in capturing the true influenza infection rates.