Deaths worldwide from the 2009 influenza H1N1 "swine flu" pandemic are likely to be nearer 280,000, some 15 times more than the 18,500 reported from confirmed lab tests, suggests a new study published in The Lancet Infectious Diseases this week.

For the study, led by the US Centers for Disease Control and Prevention, researchers developed a new model using flu data from 12 low, middle, and high income countries.

The figures they used were based on flu diagnosed from patients' symptoms and not from lab tests.

In their model they assumed that the risk of death from flu is higher in some countries that in others, and they calculated respiratory deaths associated with the 2009 H1N1 virus differently to the cardiovascular deaths linked to the pandemic.

They estimate that globally there were 201,200 respiratory deaths (ranging from 105,700 to 395,000), and 83,300 cardiovascular deaths (46,000 to 179,900) associated with 2009 pandemic H1N1 flu virus.

These figures are some 15 times higher than reported laboratory-confirmed deaths, write the authors.

"80% of the respiratory and cardiovascular deaths were in people younger than 65 years and 59% occurred in southeast Asia and Africa," they note.

The researchers suggest some problems with availability of data in the low income countries may affect the accuracy of these estimates.

However, despite these shortcomings, they conclude that a disproportionate number of estimated pandemic deaths may have occurred in lower income countries, and called for more effort to target vaccine production and supply to them in future pandemics.

In an accompanying editorial, Cecile Viboud of the National Institutes of Health and Lone Simonsen of George Washington University, suggest these estimates highlight the difficult problem of trying to keep track of a pandemic as it runs its course.

Lab-confirmed deaths hugely under-estimate the real number of deaths because there is a lack of routine testing and problems with identifying flu-related deaths, they note.

Another study in the same issue of the journal highlights the success of Scotland's vaccination campaign during the H1N1 pandemic.

In that study, researchers estimated vaccine effectiveness in a nationally representative sample of the Scottish population.

In their paper they describe how the pandemic H1N1 vaccination program started in the third week of October 2009, and by the end of January 2010, just over 38,000 people (15%) were vaccinated, leaving 85% unvaccinated.

They calculate that the effectiveness of the vaccine in preventing emergency hospital admissions from flu-related disorders was 19.5%, and the vaccine's effectiveness in preventing lab-confirmed flu was 77%.

In their conclusion they write:

"Pandemic H1N1 2009 influenza vaccination was associated with protection against pandemic influenza and a reduction in hospital admissions from influenza-related disorders in Scotland during the 2009-10 pandemic.

In August 2010, after it had been reported in over 200 countries, the World Health Organization (WHO) announced that the world is in a "post-pandemic" period with respect to H1N1 swine flu, but the virus is still circulating.

H1N1 was included in the 2011-2012 seasonal flu vaccines in the US, the UK and many other countries.

Seasonal flu kills between 250,000 and 500,000 people worldwide every year, according to WHO.

Written by Catharine Paddock