Case fatality rates (CFRs) are calculated by dividing the number of deaths from a given disease by the total number of cases of the disease. For example, if there are 2 deaths out of 100 cases, we would say that the CFR is 2%. Obviously, both the numerator (the number of deaths) and the denominator (the number of cases) are important in calculating the CFR.
The number of deaths is relatively easy to determine, assuming that you have a reasonable assay for the disease of interest. However, when it comes to influenza CFRs, we are often told that we can’t be sure of what the denominator is because we are missing mild cases. Hence, we are told that the disease is probably much less lethal than it appears because there are probably many more mild cases which have not been counted. If they were, the denominator would be larger and the CFR would thus be lower.
The claim for missing mild cases has been made for the new H1N1 virus. Is there any evidence that this is so? The only “data” presented to support this contention are some phone surveys asking people if they had respiratory symptoms during the Spring. Well, I did. I have allergies. So do a lot of other people. Without testing, there is no reason to believe that mild respiratory symptoms represent a dose of swine flu. The people who are contacted in these phone surveys did not complain to their physicians of their symptoms. Hence, they aren’t “cases”.
Several carefully controlled studies with ferrets showed that the new H1N1 virus causes more severe symptoms than seasonal flu. Hence, there is no reason to believe that a person infected with the new H1N1 virus is less likely to come to the attention of a physician than a person with seasonal flu.
When policy and advice is given on the basis of CFR, we are comparing the relative effects of the new H1N1 with respect to seasonal or past pandemic flu strains. To do this, we do not need to identify every single person who has been infected with the new virus. We just need to calculate the ratio of the people with clinically obvious disease who die. This is how the CFRs were calculated for seasonal flu and for past pandemics. During the Spanish influenza, no-one was doing seroprevalance studies or knocking on people’s doors to see if they had mild symptoms. They counted the number of people who came to the attention of doctors, counted how many died, and divided the latter by the former. If anything, we are likely understating the current CFR with respect to the 1918 flu because we are more likely to identify mild cases. People today are far more likely to go to a doctor and complain about their symptoms than in 1918, in my opinion.
If seroprevalence results were released, would we find some cases that had not been identified based on clinical symptoms? Almost certainly we would find some. It is equally likely that some deaths have also been missed. However, we do not need perfect information to compare the effects of the new H1N1 virus with previous pandemic viruses. When we apply the same methods that doctors and researchers used back then, the answer is clear.
Without mitigation, this virus will cause a severe 1918-type pandemic.