What can we learn from the H1N1 flu pandemic?

Less than one year ago, on 11 June 2009, the WHO declared the first flu pandemic in more than 40 years. Although it is not over yet, there are lessons to be learned from the information gathered during the period.

This month marks the one-year anniversary of the emergence of a new Influenza A virus, 'the swine flu', in Mexico and the United States to which the world's population had very limited immunity. In no more than a few week's time, the virus spread all over the planet, which led the WHO to declare a phase 6, or a pandemic as it is more commonly known. Declan Butler of the journal Nature looks at what we have learned from the experience. How severe has the pandemic been so far? Butler quotes Marc Lipsitch, an epidemiologist at Harvard School of Public Health in Boston, Massachusetts:

Most people were less likely to get infected than in previous pandemics, less likely to get sick if they did, and less likely to die if they got sick

However, since the pandemic is not over yet, all data has not been gathered, and it may be many years before the true extent becomes known. Research by Cécile Viboud of the US National Institutes of Health (NIH) in Bethesda, Maryland, and her colleagues suggests that the first waves may have been more severe than is generally thought. Using very conservative estimates, they think that there have been approximately 7,500 - 12,000 deaths in the US. This is less than half the annual number by seasonal influenza H1N1 and influenza B. When we look at the number of life-years lost, however, it turns out that this was around 25% higher than usual because this pandemic tended to hit harder in younger people. A less conservative estimate looked at overall mortality during the pandemic in comparison with mortality over the same period in previous years. There were 44,100 extra deaths, which is higher than in a normal flu season. Life-years lost were three to four times higher than during a virulent H3N2 season and even five times higher than for seasonal H1N1 and B viruses - the same order of magnitude as the 1968 flu pandemic. Why were older people largely spared? The seasonal flu tends to hit the very old and the very young. This was different for the 2009 season where the groups most affected were older children and young adults. More than 75% of the cases occurred in people younger than 30, and it peaked in people between 10 and 19 years old. An explanation for this phenomenon is suggested by seroprevalence studies. These monitor antibodies that react with the virus. Serum samples from England show that younger people have weaker reactions than older people, probably because older people had previous exposure to strains with similarities to the new virus. Since the pandemic virus was able to outcompete the seasonal flu strains, older people were spared a bad influenza A/H3N2 season, which would normally take a heavy toll on them. Will there be more waves of infection? This is very likely. Past pandemics have shown that they come in waves over periods of several years and that later years tend to be more severe. Lone Simonsen, a flu expert at RAPIDD, says that "We are in a pandemic period of 2 to 5 years and must continue to keep our guard up". There is a good chance that H1N1 virus will become the dominant seasonal flu strain. Even now, nearly all new flu cases are caused by the pandemic H1N1, even though there is still some seasonal influenza B virus as well, especially in Asia. Over time, more people will acquire resistance and its virulence will become no worse than that of the seasonal flu, which only causes infections when the genetics of the virus change. In the Northern hemisphere, there was relatively little flu this winter, but there are currently outbreaks of pandemic flu in tropical zones of the Americas, in west and east Africa and southeast Asia, mainly Thailand and Singapore, albeit at low levels. Winter is approaching in the Southern hemisphere and we will know soon enough whether a new pandemic wave will hit these countries. Have we learnt any lessons from the 2009 pandemic? H1N1 is a strong reminder that current techniques for making flu vaccines are too slow. It takes about six months between identifying a new virus and producing larger quantities of vaccine. This time, we had to wait until October before large amounts became available, and that was after the first wave had passed during the winter in Australia and outer Southern hemisphere countries, and several weeks into the fall wave in countries of the Northern hemisphere. Better surveillance is needed as well. Data only became available slowly and incompletely because there were hardly any seroprevalence studies being done. This was due in part to the fact that most laboratories were already overwhelmed with diagnostic samples that needed immediate processing, but there was also a lack of advance planning. Seroprevalence data are of crucial importance for making policy decisions. It is important to know how often an infection is fatal and that requires precise data on the infection rates. However, epidemiologists did not even start to get such data until September, about five months into the pandemic. There was a lag in clinical research during the pandemic, for example drug studies. Clinical researchers were slow to react because so many were busy with frontline response. There is also another problem. Anne Kelso, director of the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne, Australia: "Delays in obtaining ethical clearance was another impediment for some, with the result that the epidemic was almost over before some studies could begin."