As expected for this time of year, influenza-like illness (ILI) consultation rates have continued to increase nationwide and have surpassed the seasonal threshold. In the week ending 2 July, the ILI consultation rates and severe acute respiratory illness (SARI) hospitalisation rates were average for this time of year, but higher than in 2016.
Both Influenza A(H3N2) and B/Yamagata lineage viruses are the predominant viruses co-circulating with slightly more A(H3N2) than B viruses being detected.
The circulating A(H3N2) virus may have changed antigenically since the strains for the 2017 vaccine were selected (known as antigenic drift). The A(H3N2) virus is known to change over time more quickly than other human influenza strains, but at this stage of the season it is difficult to predict whether any changes will have an impact on the vaccine's overall effectiveness for this season or on influenza infection rates.
Also, the currently circulating B/Yamagata lineage virus is different from the lineage included in the the trivalent vaccine (which is B/Victoria). B strains do offer some cross protection, but incomplete protection is likely to be afforded against B virus infections this season.
Despite these uncertainties, immunisation remains our best prevention against influenza, and the current vaccine is recommended as the best option to provide protection. We will not know its degree of protection until the end of the season. Influenza vaccine effectiveness is analysed at the end of each season in order to evaluate its protection and inform future vaccine composition.
Other respiratory illnesses also circulate during winter that can’t be prevented by vaccination, such as rhinovirus and respiratory syncytial virus. These viruses can also cause severe disease, especially in young children and the elderly. Patients should also be reminded of ways to prevent the spread of winter illnesses by staying at home if they’re unwell, covering their nose and mouth with a tissue or inner elbow when coughing or sneezing, and washing hands regularly.
For weekly reports about influenza surveillance, see https://surv.esr.cri.nz/virology/influenza_weekly_update.php
For more information contact Andrea_McNeill@moh.govt.nz