Haemagglutinin stability was not the primary cause of the reduced effectiveness of live attenuated influenza vaccine against A/H1N1pdm09 viruses in the 2013-2014 and 2015-2016 seasons.

During the 2013–2014 influenza season, the quadrivalent live attenuated influenza vaccine (QLAIV), had lower than expected vaccine effectiveness (VE) against circulating A/H1N1pdm09 viruses in the USA. The underlying reason proposed for this was that the A/H1N1pdm09 vaccine strain, A/California/07/2009 (A/CA09), had a thermally unstable haemagglutinin (HA) protein. Consequently, a new A/H1N1pdm09 candidate strain, A/Bolivia/559/2013 (A/BOL13), was developed for inclusion in the 2015–2016 QLAIV. A key parameter for selection of A/BOL13 was its more thermostable HA phenotype compared with A/CA09. During the 2015–2016 season, QLAIV containing A/BOL13 was found in some studies to have improved, but still with suboptimal, VE against circulating A/H1N1pdm09 viruses and was not recommended for use by the CDC in the US market in the 2016–2017 influenza season. This suggested that improved HA thermostability had not entirely resolved the reduced VE observed. One hypothesis for this was that, by improving thermostability, the A/BOL13 HA protein had been over-stabilised, compromising its activation at the low endosomal pH required for successful viral entry. Here we demonstrate that, while the A/BOL13 HA protein is more stable than that of A/CA09, its thermal and pH stability were comparable with historically efficacious LAIV strains, suggesting that the HA had not been over-stabilised. Furthermore, studies simulating potential heat exposure during distribution by exposing QLAIV nasal sprayers to 33 °C for 4 h showed that, while remaining within product specification, A/CA09 viral potency was statistically decreased after 12 weeks at 2–8 °C. These data suggest that although unfavourable HA protein stability may have contributed to the reduced VE of A/CA09 in 2013–2014, it was unlikely to have affected A/BOL13 in 2015–2016. We conclude that HA stability was not the primary cause of the reduced effectiveness of LAIV against A/H1N1pdm09 viruses in the 2013–2014 and 2015–2016 seasons.