Swine flu: nearly a third of us immune

A study of the spread of swine flu during the pandemic shows that almost a third of New Zealanders (1.26 million people) had developed immunity to the H1N1 virus by the end of March 2009.

This new research indicates that some 480,000 people had immunity prior to last year’s pandemic wave – mostly in older age groups. This appears to confirm that lower infection rates among the elderly were due to historical exposure to a similar virus.

Transmission and infection impacted the young most severely, according to the study, which found that infection among school children was around 10 times higher than expected. One in three school-age children were infected, nearly half with no obvious symptoms.

The research was undertaken by Environmental Science and Research and commissioned by the Ministry of Health. The study is available for download here.

The SMC wrapped up comment from pandemic experts.

Dr Lance Jennings, Clinical Associate Professor, Virology & Serology Department, Canterbury Health Laboratories comments:

“This information is helpful in that it indicates that a large proportion of New Zealanders remain susceptible to the H1N1 2009 virus. Influenza is a serious disease and our best protection from it is through immunisation. Influenza vaccine is available free to those at greatest risk from infection, and to others who wish to protect themselves from influenza until 30th June.”

Professor John Fraser, Head of School of Medical Sciences, Maurice Wilkins Centre for Molecular Biodiscovery, University of Auckland comments:

“This is a well performed study that highlights some extraordinary features about the 2009 H1N1 influenza strain; notable the extremely high rate of infection in just a few months during the winter of 2009, the low pathogenicity (far fewer deaths than expected) and most extraordinary almost half the positive tests were from asymptomatic carriers. This latter feature is likely to be the main reason for the very high infection rate; people transmitting the virus without knowing that they were infected.

“The key unanswered question remains to what degree do the serum antibodies reflect protection. Nearly half of all NZ children aged 5-19 (the group most vulnerable to infection) now have antibodies to the 2009 H1N1 strain after only a single season.

“This substantial herd immunity should slow the progress of future waves but only if those antibodies are protective. There are of course the other 50% of children who remain at risk.

“People should not be complacent. They should seek the simple option of the available vaccination. That is the only way we can ensure the community is well protected. The nightmare scenario would be the return of a mutant strain with a similar transmission rate but much more pathogenic.”

Dr Michael Baker, Associate Professor, Department of Public Health, University of Otago, Wellington and a contributor to the ESR study responds to SMC questions:

1. Is this level of immunity in the population higher or lower than expected?

“I thought the really interesting finding from this study was the relatively high level of immunity to this virus that was already present in the NZ population before the pandemic, particularly in older age groups. As the report notes, an estimated 12% of the population were already immune to this virus, which suggests about 18% of the population were infected by the new pandemic virus in 2009 (resulting in total immunity of 29.5%).”

2. What does it say about how the virus spread and how it affected people?

“Findings from this report help to explain why the epidemic spread rapidly in New Zealand in June 2009, and then transmission stopped quite abruptly with far fewer clinical cases than were expected. And also why few older people were infected. This pattern was caused by the relatively high levels of existing immunity in older age groups, and also by the relatively mild nature of infection meaning that many infected people had mild or absent symptoms.

3. Have the public become overly complacent about the pandemic in your view? How should these findings inform people’s responses?

“Results of this study are a powerful reminder of how immunity can protect vulnerable people. The elderly population of NZ was largely protected from the serious effects of last year’s pandemic because of good fortune – previous influenza viruses, particularly from before 1957, gave them some protection. Younger people with certain health conditions, such as pregnancy, are particularly vulnerable to serious illness and death from this virus. They should consider protecting themselves using flu vaccine.

“The Ministry of Health and ESR should be congratulated for funding and carrying out this study. This is one of the few national serological studies done following the pandemic and is a major accomplishment. It makes an important contribution to our understanding of the behaviour of influenza pandemics.”

Dr Baker has contributed to an separate paper on the transmission of swine flu during long-haul flights which appeared this week in the British Medical Journal and is available on their website.

Further Information

For more information, or to contact any of the experts above, contact the Science Media Centre on tel: 04 499 5476 or email: smc@sciencemediacentre.co.nz.