Early vaccination could stifle swine flu in the US

Researchers say a vaccination programme starting in September, reaching 70 per cent of the US population and targeting children first, could effectively curtail the H1N1 pandemic as the US moves into winter.

vaccineThe authors of the paper, published in Science tomorrow, used a computer model and up-to-date information about the pandemic to simulate how it could spread in the coming months under various conditions.

They found that H1N1 is highly transmissible and that starting vaccination of children is essential as each infected child has the potential to infect two to three others around every three days.

While the number of swine flu cases in New Zealand appears to be abating, the virus is still circulating and health officials are planning for the release here of a vaccine in preparation for next year’s flu season.

The Science Media Centre rounded up comment from experts on the significance of the research and what New Zealand can learn from it.

Registered journalists can access the research in our Resource Library.

Dr Sue Huang, Head, WHO National Influenza Centre, Institute of Environmental Science and Research and member of the Australian influenza Vaccine Committee, comments:

“The Science paper is very interesting. Estimation of the transmissibility of the pandemic H1N1 2009 virus is very important in order to plan effective vaccination strategies. NZ’s surveillance data;  also indicated high transmissible nature of this virus: the pandemic (H1N1) 2009 virus became the predominant strain within just one month in New Zealand. This will be one of the important factors for consideration for the upcoming WHO meeting regarding the vaccine composition for the southern hemisphere for next year.

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

“This is a very good paper and makes some interesting observations and predictions which I have outlined below:

1.       The H1N1 is more transmissible than previous strains with a 27% household attack rate (probability of being infected by a family member).

2.       The case fatality rate (0.2%-0.68%) from the virus does not appear to be much greater than normal seasonal flu.

3.       The main routes of infection initially are schools and households – hence the 5-18 year age group shows the highest levels of infection at the start.

“Using this data, the authors model the rates of spread and then apply some predictive vaccination scenarios.  They show that to completely prevent a pandemic, pre-vaccination needs to be at a level of >50% coverage. Pre-vaccination is important in order to allow the immunity to develop.

“Their model also suggests that if phased immunisation is to be adopted, children should be vaccinated first as this will provide the greatest delay and offer the greater community protection.

“The real problem is that the vaccination regime recommends two inoculations about 30 days apart so there will be issues of compliance and I am not sure whether the effect of the 30 day delay has been modelled in – I presume that is the difference between the pre-vaccination data and the on-demand model.

“There is no reason why NZ should not adopt this vaccination strategy but it will come down to cost and mobilisation.  In the US, the issue is more critical because they are heading into their winter.”

Dr Anne La Flamme, Senior Lecturer in Immunology and Cell Biology, Victoria University, comments:

“Quite an interesting article. It gives some hope that the pandemic can be avoided in a ‘second wave’ when the Northern Hemisphere enters its flu season. First, the article confirms (not their data) that the novel H1N1 strain seems to be fairly stable so that the current vaccine being produced is likely to induce effective immunity. Good news. Then they explore possible 2 vaccination questions: 1) how many need to be vaccinated for ‘herd immunity’ and 2) if they cannot vaccinate everyone at once and before infections start (most likely scenario), how should they plan to vaccinate.

“For 1) if they can get at east 70% vaccinated, a pandemic can be avoided even if the virus is highly transmissible. Given their estimate of transmissibility of 1.6 (1 person infects 1.6 others), even coverage of 50% will completely stop the pandemic and 30% reduces it significantly. However, one issue is that the current estimates in New Zealand (Baker and Wilson, WSM, Wellington) show the transmissibility to be closer to 2. The difference may be that we are in our flu season and the US isn’t. However, if 2.0 is used in their calculations then they will need to vaccinate at least 70%. Note that these are all based upon prevaccination and everyone at once.

“For 2) they calculate that while vaccination before the pandemic begins is most effective, they can also phase in the vaccination over a 4 month period and that this strategy is most effective if they start with children and then target adults.

“I think that the key to all of these calculations and models is the level of transmission (what they call Ro in the article). If it is 1.6, as they estimated in the US, their vaccination plans should be very successful. However, if it is closer to 2.0 as we found in New Zealand, their strategies will be much less effective at reducing the overall illness attack rate (# ill). It may delay or spread the peak, which would help the health services cope better with the numbers. Instrumental in lowering the transmissibility are public health measures to prevent the spread. Thus, vaccination may prevent a pandemic but probably not without maintaining stringent public health measures to reduce the spread.”

Professor Kurt Krause, Director of the Webster Centre for Infectious Diseases, University of Otago, comments:

“The idea of vaccinating the population of any country to prevent a second wave of H1N1 infection is an extremely important issue. Making the correct decision here requires that we balance the risk of natural infection, and any mortality and morbidity from the natural infection, with the small risks inherent in any major vaccination program.

“I would recommend that all customary precautions that are applied to the testing of seasonal flu vaccines be applied to novel H1N1 formulations before they are put into use. As an aside, I also would note that T. Garske et al., was not, as the authors imply, actually a reference that endorses the higher case fatality rates found in countries like Mexico, rather it was an analysis of the risks that those numbers being reported could in fact be inaccurate coupled with analysis and discussion of what steps might be needed to avoid calculating these results in error in the future.”

Further Information
To talk to these experts, please contact the Science Media Centre on tel: 04 499 5476 or email: smc@sciencemediacentre.co.nz.