Better estimates of A(H1N1) flu virus severity needed, say experts

[Chuck]

http://www.bjhcim.co.uk/news/2009/n907029.htm

Better estimates of A(H1N1) flu virus severity needed, say experts

14 July 2009

Accurate estimates of the severity of the new H1N1 virus, and in particular how many deaths might arise over the course of the pandemic, are central to healthcare planning over the coming months, say experts in a paper published on bmj.com today.

They will also help to influence decisions on whether to implement social distancing measures such as school closures.

At first sight, the data appear to imply that this new Influenza A (H1N1) virus is relatively mild, with case fatality ratios (the total number of deaths due to the disease divided by the total number of cases) around 0.5%, similar to the upper range of that seen for seasonal influenza, and relatively low hospitalisation rates.

However, severity appears to vary substantially between countries and fatal cases have been much younger than for seasonal influenza.

So researchers at Imperial College London analysed the difficulties in assessing the severity of the new virus. They report that in most infectious diseases there is a risk of bias towards diagnosis of more severe and hospitalised cases, which overestimates the case fatality ratio.

In contrast, some deaths caused by flu might not be recognised as such, as flu infections can temporarily increase the risk of vascular death, such as heart attacks and strokes. This causes an underestimation of the case fatality ratio.

Another important source of bias arises from the time delay between disease onset and death, which in the early phase of an epidemic can lead to an underestimate of the case fatality ratio, they add.

In order to get a clear picture of the severity of the H1N1 virus, it is vital to take these and other factors into account, say the authors. They propose study designs and statistical analysis methods to improve our ability to obtain reliable case fatality ratios.

Critically, they will ensure that any changes in the virulence of the virus are rapidly detected so that mitigation policies are applied appropriately, they conclude.

Dr Tini Garske, lead author of the study from the Medical Research Council (MRC) Centre for Outbreak Analysis & Modelling at Imperial College London, said: "Accurately predicting the severity of this swine flu pandemic is a very tricky business, and our research shows that this can only be achieved if data is collected according to well designed study protocols and analysed in a more sophisticated way than is frequently being performed at present.

"If we fail to get an accurate prediction of severity, we will not be providing healthcare planners, doctors and nurses, with the information that they need to ensure they are best prepared to fight the pandemic as we head into the flu season this autumn."

 

[Desolation_anonymous]

Re: Better estimates of A(H1N1) flu virus severity needed, say experts

Exactly.

Can some explain to me, are the 'case fatality rates' accurate? So far, I've seen, .5%... BUT...

For instance, US lists an overall CFR of .5% if you take the deaths and 'confirmed and probable cases'... from the CDC

But we know that most states are just testing those severely ill, and often just those hospitalized...

(Had to remember this at first, because without remembering CA would be 1% cfr)!

And if "thousands are likely infected in San Francisco alone', I doubt the 2000+ cases in California is accurate...?

But I also know that there is the issue of deaths being reported after the fact, deaths being counted the same time as all cases (instead of from time of illness, etc.)

Based on the above variables, it would appear that CFR in the US is likely under .5%? But if so, what is it?

Is there any data that gives us a clear picture?

 

[Mamabird]

Re: Better estimates of A(H1N1) flu virus severity needed, say experts

Exactly.

Can some explain to me, are the 'case fatality rates' accurate? So far, I've seen, .5%... BUT...

For instance, US lists an overall CFR of .5% if you take the deaths and 'confirmed and probable cases'... from the CDC

But we know that most states are just testing those severely ill, and often just those hospitalized...

(Had to remember this at first, because without remembering CA would be 1% cfr)!

And if "thousands are likely infected in San Francisco alone', I doubt the 2000+ cases in California is accurate...?

But I also know that there is the issue of deaths being reported after the fact, deaths being counted the same time as all cases (instead of from time of illness, etc.)

Based on the above variables, it would appear that CFR in the US is likely under .5%? But if so, what is it?

Is there any data that gives us a clear picture?

It is very difficult to know any of these things with certainty since reporting varies widely across regions. However, you would normally look at several things:

1. What is the percentage of the population that shows up at the clinic with influenza-like illness. That is the Clinical Infection Rate. For seasonal flu that varies from 5% to 15% year to year. For Swine Flu, that number could either be unreasonalbly high or low depending on how the population reacts. If the symptoms are really mild, many thousands of folks may be ill, just not sick enough to go to the doctor. On the other hand, we have seen some panic in many areas, and folks that are not really that sick show up at the clinic out of fear.

2. What is the percentage of those showing clinical signs of influenza that are hospitalized. For seasonal flu that number is around 200K hospitalizations each year on average. I don't think we have seen anywhere near that rate of hospitalizations for Swine Flu, but the Pandemic is still young.

3. What is the percentage of those showing clinical signs of influenza and pnemonia that die. This is the Case Fatality Factor. Again, on average some 36K people die in the US each year due to sesonal flu. Now we are not seeing anywhere near that level of fatalities with Swine Flu, but the disconcerning fact is those that don't recover are relatively young. Most seasonal flu deaths are the over 65 crowd.

Having said that, many will dispute all of these numbers as being mere guesstimates and not very useful. However, what is useful is trends from week to week. So, monitor hospitalizations and deaths week to week due to influenza like illness and draw you own conclusions about the pathogenicity of the Swine Flu in its current state. Avoid reported cases, or confirmed cases of Swine Flu, since the variability of those numbers is substantial and rather meaningless.

 

[somebodyoutthere]

Re: Better estimates of A(H1N1) flu virus severity needed, say experts

Here's a research study that shows a CFR of .4% - is it credible? Why the discrepancy?

http://www.radionz.co.nz/news/stories/2009/07/11/1245bbb31b6a

Professor Mick Roberts of Massey University's institute of information and mathematical sciences is warning people not to underestimate the seriousness of the disease.

He says that early reports suggested lower swine-flu death rates in the United States and Canada than in Mexico, where the virus first broke out.

But that, he says, has been disproved by a Dutch research project with which he has been involved. It shows that about 0.4% of all confirmed cases in the US, Canada and Mexico have been fatal.

 

[LMonty]

Re: Better estimates of A(H1N1) flu virus severity needed, say experts

what i think we might eventually see is a bump in the total weekly or monthly morbidity and mortality figures. i expect to see them scew to an uptick in pneumonia deaths first.

for example, we have seen dozens of out of season pneumonia cases lately, some quite severe. many in the elderly, that have started with flu like illness. my guess is they are secondary infections and we have been spared most of the ARDS type response in the young so far. I recall at least one PNE death recently. AFAIK what few MDs are testing for flu are testing for Influenza A and B only, not requesting H1N1 testing. Ive heard at least a dozen times at the hospitals in the last few weeks about these odd out of season pneumonias and flu like symptoms.

I think we are only seeing the tip of the iceberg of mortality and morbidity so trying to come up with figures for that right now is a lost cause. methinks we just arent collecting data.