Influenza A H1N1 in New Zealand – Experts respond

A number of cases of Influenza A virus subtype H1N1 (commonly dubbed swine flu) have been recently reported in New Zealand.

Most recently an outbreak of eight cases in Hawke’s Bay has received widespread coverage.

While it is not unusual for the H1N1 strain to be detected in flu cases, experts have noted that it is uncommon to see so many cases in the warmer months.

The SMC collected the following expert commentary.

Dr Sue Huang is a virologist at ESR and Director of New Zealand’s World Health Organisation (WHO) National Influenza Centre. She comments:

“Every year the Influenza virus travels around the world and constantly mutates which makes it very difficult to predict.

“ESR works with other partners in the health sector to take an active role in the global research and response to influenza and provide New Zealand with the highly effective surveillance systems it needs to stay on top of response measures.

“Everyone needs to pay close attention to warning signs and not treat every onset of sickness the same. People need specialist help if they are suddenly taken ill with symptoms that include a high temperature (over 38 degrees) and coughing.

“School aged children are particularly vulnerable to getting the flu, and because of the many circles they tend to move in, are also highly likely to pass it on to others in home and community .

“Other points to note:

  • Vaccination is the most effective tool to prevent influenza related illness.
  • Swine flu has historically been a predominant strain in Canada and the United States, comprising more than 90 percent of detected influenza viruses but this year’s season was relatively normal.
  • It’s not certain that NZ would also see more influenza A(H1N1)pdm09 viruses circulating, but we expect to see co-circulation of A(H1N1)pdm09, A(H3N2) and B viruses which are covered by the 2014 influenza vaccine composition.
  • A recent serosurvey conducted in Canada in May 2013 showed that seroprotective antibody to A/California/7/2009 (H1N1)pdm09 was high among school-aged children and elderly; however, seroprotection was lower among very young children and adults between 20-69 years. The resurged A(H1N1)pdm09 virus in 2013/2014 season in Canada caused more infections in this age group. The low level of pre-existing antibody toward A(H1N1)pdm09 in this age group may explain such a demographic shift in disease burden in Canada.
  • Virological investigation showed that the circulating A(H1N1)pdm09 virus in North American were antigenically similar and genetically conserved and very closely related to the vaccine strain (A/California/7/2009 (H1N1)pdm09).  Based on the northern hemisphere influenza circulation pattern, WHO recommended the following strains for Northern Hemisphere for their incoming winter (2014/2015). They are the same as the 2014 vaccine composition for New Zealand:
    • an A/California/7/2009 (H1N1)pdm09-like virus;
    • an A/Texas/50/2012 (H3N2)-like virus;
    • a B/Massachusetts/2/2012-like virus.”

Associate Prof Lance Jennings, Clinical Virologist, Canterbury District Health Board, comments:

Has the North American flu season been particularly severe this winter, and is swine flu playing a strong role in this?

“Influenza activity during the 2013-14 Northern Hemisphere winter has been high in United States and Canada, and interestingly moderate in Europe, and moderately high in Japan and China. Influenza A(H1N1)pdm09 has been the predominant virus in the USA and Canada, the first season that this virus has circulated at high levels since 2009 (74% of A identifications H1N1). More mixed activity due to H1N1 and B has been seen in Japan, China & Europe. B/Yamagata lineage viruses have been the predominant B virus in the United States and Europe. ”

Who is likely to have immunity from the H1N1 strain at this stage? Which parts of the population are most vulnerable?

“The immune status of individuals in New Zealand is not routinely assessed, however, in the United States this last season, 60% of the influenza associated hospitalisations have been reported in the 18-64 year age group, a trend of hospitalisations among younger people also seen during the 2009 H1N1 pandemic. Confirmed H1N1 cases in 2014 in New Zealand have largely been in this age band.” 

How important/effective is annual immunisation in minimising impacts from the disease?

“Estimates of seasonal influenza vaccine effectiveness (VE) for preventing medically attended illness caused by H1N1pdm09 influenza viruses have ranged from 50% to 80% in previous seasons.  For the North American 2013014 season, overall VE for all ages against influenza A and B virus infection associated with medically attended acute respiratory illness was 61% (95% confidence interval = 52%-68%). Against the predominant influenza pH1N1 virus, VE for all ages was 62%, with similar protection from medically attended illness across age groups. (report here).

“Annual seasonal influenza vaccination provides the best protection against influenza, and the 2014 influenza vaccine should provide excellent protection against the H1N1 virus and H3N2 and B Yamagata viruses likely to circulate in New Zealand this coming winter.

“Influenza vaccine is free for pregnant women, indivuals 65 years and over, children from 6 months of age and adults with certain underlying medical conditions ( and in Canterbury, children under the age of 18 years. Anyone wishing to protect themselves against influenza should talk to their general practitioner or pharmacist about receiving the vaccine.”