Vaccine effectiveness in the 2011-2012 season was modest overall, with lower effectiveness against the predominant A (H3N2) virus. This may be related to antigenic drift, but past history of vaccination might also play a role.

Background. During the 2012–2013 influenza season, there was cocirculation of influenza A(H3N2) and 2 influenza B lineage viruses in the United States.Methods. Patients with acute cough illness for ≤7 days were prospectively enrolled and had swab samples obtained at outpatient clinics in 5 states. Influenza vaccination dates were confirmed by medical records. The vaccine effectiveness (VE) was estimated as [100% × (1 − adjusted odds ratio)] for vaccination in cases versus test-negative controls.Results. Influenza was detected in 2307 of 6452 patients (36%); 1292 (56%) had influenza A(H3N2), 582 (25%) had influenza B/Yamagata, and 303 (13%) had influenza B/Victoria. VE was 49% (95% confidence interval [CI], 43%–55%) overall, 39% (95% CI, 29%–47%) against influenza A(H3N2), 66% (95% CI, 58%–73%) against influenza B/Yamagata (vaccine lineage), and 51% (95% CI, 36%–63%) against influenza B/Victoria. VE against influenza A(H3N2) was highest among persons aged 50–64 years (52%; 95% CI, 33%–65%) and persons aged 6 months–8 years (51%; 95% CI, 32%–64%) and lowest among persons aged ≥65 years (11%; 95% CI, −41% to 43%). In younger age groups, there was evidence of residual protection from receipt of the 2011–2012 vaccine 1 year earlier.Conclusions. The 2012–2013 vaccines were moderately effective in most age groups. Cross-lineage protection and residual effects from prior vaccination were observed and warrant further investigation.

NCT00538512.

Three type A influenza viruses, each of which has a distinct neuraminidase-gene mutation and is resistant to the neuraminidase inhibitor oseltamivir, have been isolated. Previously, in the ferret model, an R292K mutant of a type A (H3N2) virus was not transmitted under conditions in which the wild-type virus was transmitted. This model was used to investigate whether the E119V mutant of a type A (H3N2) virus and the H274Y mutant of a type A (H1N1) virus would be transmitted under similar circumstances. Both mutant viruses were transmitted, although the H274Y mutant required a 100-fold-higher dose for infection of donor ferrets and was transmitted more slowly than was the wild type. Both the mutant and the wild-type viruses retained their genotypic characteristics.

In the 2004-2005 season, in which most circulating viruses were dissimilar to those included in the vaccine, the inactivated vaccine was efficacious in preventing laboratory-confirmed symptomatic illnesses from influenza in healthy adults. The live attenuated vaccine also prevented influenza illnesses but was less efficacious. (ClinicalTrials.gov number, NCT00133523.)