Reproduction number 1.96 - 79% of population could be affected

[kiwibird]

Infectiousness Of Swine Flu Estimated For New Zealand


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Friday, 24 July, 2009 - 10:40

University of Otago, Wellington researchers say that the pandemic influenza virus appears more infectious than previously thought. The first estimate for New Zealand suggests that a person who is ill with this new strain will on average infect almost two others.

The transmission potential of a virus in a pandemic is typically summarised by the reproduction number. This measures the average number of secondary cases generated by a single primary case.

Associate Professor Michael Baker and Dr Nick Wilson from the Department of Public Health at the University of Otago, Wellington have detailed the latest estimate of the spread of the new virus in today's New Zealand Medical Journal.

This work was done in collaboration with Dr Hiroshi Nishiura, a mathematical modeller based at the University of Utrecht in Holland. It gives the first published estimate for the reproduction number of the pandemic in the Southern Hemisphere.

"Our best estimate of the reproduction number for the Influenza A virus in New Zealand is 1.96, which is somewhat higher than the number we have previously used in modelling estimates," says Associate Professor Baker. "To date we have tended to use a lower estimate of 1.5 which was published early in the pandemic based on data from Mexico."

A reproduction number of 1.96 means that up to 79% of an affected population could ultimately catch H1N1 influenza during the epidemic (though only two-thirds of those infected may be expected to show symptoms). However, that would require a high level of mixing between groups. Effective public health interventions could lower that proportion.

Published estimates of the reproduction number have ranged from 1.4-1.6 in Mexico to 2.0-2.6 in Japan in the current influenza pandemic. New Zealand has a higher reproduction number than Mexico, possibly because of the winter season and large clusters of cases in certain settings.

However the researchers say that further work will be needed to produce more robust estimates for New Zealand. This current estimate could be reduced after more accurate demographic analysis, and if public hygiene behaviour and health interventions are effective.

"This result reinforces the need for those with influenza symptoms to do everything they can to prevent infecting others," says Dr Nick Wilson. "Staying away from work and school while ill and practicing good 'cough etiquette' can all help to lower the spread of this new virus in New Zealand".

The letter to the New Zealand Medical Journal points out that the current estimated reproduction number is higher than that of the Spanish influenza pandemic of 1918-19, but the mortality rate is very low compared to that disaster, in which thousands died.

Earlier this month in a separate published study, Associate Professor Baker and Dr Wilson estimated that the mortality rates in developed countries from this pandemic could be as low as one death out of 10,000 cases for this new pandemic strain.

http://www.voxy.co.nz/national/infectiousness-swine-flu-estimated-new-zealand/5/19105

[FrenchieGirl]

New Zealand - Swine flu more infectious than 1918 flu

Swine flu more infectious than 1918 flu, study finds

11:42AM Friday Jul 24, 2009
By Martin Johnston

File photo / Martin Sykes



Swine flu is more infectious than both seasonal influenza and the 1918 Spanish flu, a study based on New Zealand epidemic case numbers has found.

And this suggests nearly 79 per cent of an affected population will catch the virus, although not all will become ill.

The researchers, from Otago University at Wellington and Utrecht University in the Netherlands, say in a letter published in today's New Zealand Medical Journal that it was estimated early in the swine flu pandemic that the transmission rate was around 1.5. This means that every two people who are sick with the virus will infect three others.

This was calculated in Mexico, where the virus first emerged.

But Associate Professor Michael Baker, Dr Nick Wilson and Dr Hiroshi Nishiura estimate from New Zealand data that every case is infecting nearly two others - a rate of 1.96.

This is based on official notifications of swine flu between June 2 and 15. This was a period of exponential growth in the New Zealand epidemic. It began with the first case of local transmission, excludes 63 imported cases, and ends before health authorities, because so many people were catching the virus, gave up testing every potential case.

In Japan a transmission rate of between 2 and 2.6 was calculated.

The Medical Journal letter notes that a transmission rate of 1.96 is higher than published estimates for seasonal influenza in temperate-zone countries like New Zealand. It is also "slightly greater than that of Spanish influenza pandemic from 1918-19 in New Zealand", but the death rate from Spanish flu was much higher than in the current swine flu pandemic.

Drs Baker and Wilson have estimated the death rate from swine flu in developed countries could be as low as one death out of 10,000 cases.

The letter says a transmission rate of 1.96 suggests that 78.6 per cent of the population will experience infection by the end of the pandemic, although only two-thirds of those infected are expected to show symptoms.

But public health interventions like promoting good hygiene and social distancing were likely to reduce the proportion infected to below that estimate.

This is the first swine flu transmission rate estimate for the southern hemisphere. The researchers say the reasons it is higher than the Mexican estimate include that the south is now in winter and flu viruses spread more readily between people in wintertime.

The Ministry of Health said yesterday the official death toll from the swine flu pandemic stood at 11. However, chief coroner Judge Neil MacLean said he was investigating the deaths of a further three people who had the virus.

The ministry said the number of confirmed past and current cases - now just a fraction of the likely total number - was 2525.

[Sally Furniss]

Re: Reproduction number 1.96 - 79% of population could be affected

Swine Flu Sufferers Pass Bug to at Least Two Others, Study Says


By Jason Gale
July 24 (Bloomberg) -- Swine flu sufferers pass the bug to about two other people, fostering its spread, according to the first published study of the pandemic strain’s infectiousness in the Southern Hemisphere.
Researchers in New Zealand and the Netherlands used mathematical modeling to estimate the transmission potential of the new H1N1 strain, also known as swine flu. In New Zealand, where winter weather is fanning the worst flu season in more than a decade, the virus may be spreading more easily than it did in Mexico, where it emerged more than four months ago.
The research, published in today’s New Zealand Medical Journal, may help public health officials in North America and Europe anticipate what to expect when the pandemic virus circulates there next winter. The reproduction number in New Zealand is 1.96, meaning that up to 79 percent of people could potentially catch the germ during the epidemic, said Michael Baker, one of the study’s authors.
“To date we have tended to use a lower estimate of 1.5 which was published early in the pandemic based on data from Mexico,” Baker, who is an associate professor of public health at University of Otago in Wellington, said in a statement.
Published estimates of the reproduction number have ranged from 1.4 to 1.6 in Mexico, and 2.0 to 2.6 in Japan. New Zealand has a higher reproduction number than Mexico, possibly because of the winter season and large clusters of cases in certain settings, the authors said.
Winter Bug
England is suffering the worst bout of flu in a decade and the number of swine flu cases in the U.K. rose by 100,000 in the past week, Britain’s Chief Medical Officer Liam Donaldson said yesterday.
The pandemic bug appears to be as contagious as seasonal influenza, and is spreading faster among people aged 10 to 45 years, according to the World Health Organization. The severity of the disease ranges from mild symptoms such as sore throat and muscle ache to severe illnesses including pneumonia that can result in death, WHO said.
Further studies are needed to produce more robust reproduction estimates for New Zealand, the authors said. Effective public health interventions, including improved hygiene-related behavior, could stem the spread of the disease, said co-author Nick Wilson.
“Staying away from work and school while ill and practicing good cough etiquette can all help to lower the spread of this new virus in New Zealand,” Wilson said in the statement.
Even though the estimated reproduction number is higher than that of the Spanish influenza pandemic of 1918, the mortality rate is much lower, the authors said. The Spanish flu pandemic killed more than 40 million people worldwide.
Mortality rates in developed countries from the current pandemic could be as low as one death out of 10,000 cases, Baker and Wilson said in a separate study earlier this month.
To contact the reporter on this story: Jason Gale in Singapore at j.gale@bloomberg.net.
Last Updated: July 24, 2009 02:46 EDT

http://www.bloomberg.com/apps/news?pid=20601124&sid=adrAANGzz3r4