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The direct mortality of H1N1 would be a hundred times that of seasonal influenza

Posted on August 27th, 2009 in swineflu H1N1

While the northern hemisphere prepares to confront the pandemic in the fall, the spread of influenza A (H1N1) in the southern hemisphere to better estimate the aggressiveness of the new virus. In New Caledonia, the record was Wednesday, August 26 morning, five deaths directly caused by the virus in a population of 250 000 inhabitants, probably more than 35 000 people had contracted the virus …
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Two suspicious deaths in Reunion

Two dead, “possibly related” to the influenza A (H1N1) were recorded at the meeting, announced Wednesday, August 26th prefecture. It would in this case the first two deaths on Réunion in connection with the epidemic of H1N1 influenza.

People died, aged 90 and 60, suffered severe chronic diseases associated with influenza-like illness.

According to estimates released Wednesday, 22 500 people have been affected by the epidemic in the department from July 5 to August 23, in a population of 800 000 inhabitants.
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Antoine Flahault, professor of medicine and specialist in flu, the World Book’s latest analysis of epidemiological data and dispelling misconceptions about the risk groups, vaccination strategies and use of Tamiflu.

What do the recent epidemiological data on the virulence of H1N1?

The virulence of an emerging epidemic disease – that is to say, his aggressiveness – is one of the most difficult parameters to evaluate. What to measure? Complications of influenza, hospitalizations, or mortality? The latter is the least dependent on national parameters such as quality of information systems, the hospital device, etc.. But things are complicated: there are three ways to die from the flu. The first, disappearing in developed countries, is bacterial superinfection. The second is a viral pneumonia that causes a syndrome of acute respiratory distress (ARDS) and death in 50% to 60% of cases. In this situation of “direct mortality”, the virus that kills. The third leading cause by far the most common is the worsening of preexisting severe illnesses. This mortality, which particularly affects the elderly, has become almost incompressible for about twenty years. For seasonal flu, it is about one death per 1 000 infections in rich countries.

For influenza A, which of these mortalities is it fear?

It is often said that influenza A would tend to kill more young people and fewer older people. But it may be a little less hasty.

In the case of seasonal flu, the death of the third kind – those that disproportionately affect the elderly – are a death “invisible” at the time. It is detected in statistics, often several months after the epidemic. Each year after the passage of influenza seasonal mortality is increased about 5 000 to 6 000. For this particular form of mortality, the effects of pandemic influenza should not be lower than the seasonal flu. But it is too early to detect.

However, 50% of cases of Influenza A affecting individuals under 20 years …

True, but it is also for seasonal influenza? In France, 50% of cases of seasonal influenza occur also in less than 20 years.

There is no class of persons “at risk”?

The mortality that we “see” today is essentially direct mortality due to ARDS. But these deaths seem to occur on an almost random in the population affected by the virus, so most often in young people. In the case of seasonal influenza, very few cases of ARDS are reported and we can not compile statistics to say that one group is more vulnerable than another. The Centers for Disease Control and Disease Prevention in Atlanta have been published this description of ten cases of influenza A ARDS deaths, nine were obese. This severe obesity might represent a potential risk factor, which was previously unknown. Similarly, 13% of deaths from influenza A in the United States occurred in pregnant women, most of them died of ARDS. They could also represent a population at risk.

What is the frequency of direct mortality by ARDS?

For seasonal flu, it can be estimated at one case per million or less. In France, we average 5 to 6 million cases of seasonal flu each year, and to my knowledge, no more than 5 to 6 cases of ARDS deaths from the disease.

For influenza A, we now have situations better documented. In New Caledonia, when it was in 3 cases [5 cases are now documented] the government estimated cases of influenza A to 20 000 cases – so probably about 30 000 infections, counting cases of “silent”. In Mauritius, there were 7 deaths in intensive care units of 15 000 cases reported, but my contacts with the medical community of the island make us think they are more numerous, probably 50 000 cases, so 70 000 infections. It is facing a disease that direct mortality – one case per 10 000 infections – is certainly very rare but still 100 times higher than the seasonal flu. We have just published in PLoS this Currents Influenza.

A mass vaccination you seem appropriate?

It has never attempted to use the vaccine as a barrier against an epidemic of influenza, even seasonal. With the exception of Japan in the years 1970 to 1980 and the United States in 1976.

The only strategy that now has solid experience in the field of influenza is the protection of persons at risk. The knowledge of this virus could change the definition of these groups, first and always the elderly or vulnerable populations, health professionals and finally the new categories that I mentioned: the large and obese pregnant women.

There is therefore no reason to change this strategy?

Unfortunately though, there would be many reasons, theoretical, we have just published in BMC Infectious Diseases. A strategy of mass immunizing third of the world’s population, would have barred the road to pandemic. But I do not think we can propose such a strategy based solely on results of mathematical models, without trying to test its validity in the population. It would be a sort of adventure to try to engage in a completely innovative strategy vis-à-vis the influenza A when it has not only been attempted on the seasonal flu.

They could face any side effects? We can not exclude it. After a campaign of mass immunization, disease occurrence in the weeks that follow might be even falsely attributed to vaccination. We saw this for the vaccine against hepatitis B lorsqu’étaient occurring cases of multiple sclerosis: a person is able to determine their origin. This has led to a lasting stigma on immunization.

What Tamiflu?

Means that it should be reserved for serious or complicated cases, which surprises me a bit because the only available clinical trials have not focused on the prevention of complications or mortality. Its use has primarily a collective aim, because it is effective to reduce viral load, duration of symptoms and therefore the circulation of virus in the population. Tamiflu is also effective preventive treatment. There is still some resistance, so it’s time to use it, because then he might be permanently useless.