Tracing changes in the impact of swine flu

The latest figures for patients hospitalized in England for suspect swine flu provide an opportunity to compare the current cases with those in the first wave in July.

Table 1 compares the numbers by age-group, and the subtotals in intensive care on 28 July 2009, the first date for which Health Protection Agency’s (HPA) weekly reports provided the data, and on 18 November 2009, the most recently released HPA data.

The age-distribution of patients in intensive care has changed, but the change is not quite statistically significant (chi-square on 3 degrees of freedom is 7.4; the 5 per cent critical value is 7.9). But the change in the age distribution for all inpatients with suspect swine-flu most certainly is significant (chi-square on 3 degrees of freedom is 25.1).
Relatively more inpatients with suspect H1N1 on 18 November 2009 were under 5 years of age and many fewer were aged 65+ years. Has vaccination with seasonal flu vaccine benefited the 65+ age-group?
There is no indication from virologists that H1N1 has evolved to become virulent. Yet, for roughly the same number of inpatients, more than twice as many patients were in intensive care on 18 November as on 28 July. Has the index of suspicion for swine-flu changed so that more inpatients were designated as ‘suspect swine-flu’ in the summer than are now? If so, did any change in the index of suspicion apply equally across all age-groups?

Alternatively, the two dates may represent slightly different stages (at/after peak) of the summer/autumn epidemic of H1N1, which could be expected to influence the proportion of inpatients who are currently in intensive care. These data, and the unresolved questions they generate, help to explain why it is essential that all hospitalized patients who are designed as ‘suspect’ swine-flu are actually tested for H1N1.

The HPA’s weekly report does not give the age-distribution (as above) for cumulative swine-flu deaths in England nor does the Chief Medical Officer’s weekly report separate out deaths in the under 5s from the under 15s. However, as shown above, the under 16s represented about a tenth of the patients in intensive care units for swine-flu - both on 28 July and on 18 November 2009.

Scotland has reported 38 deaths from swine-flu to16 November 2009, of whom only two were under 15 years of age (30 were aged 15-64 years and 6 aged 65+ years). Roughly one quarter of Scotland’s hospitalizations for confirmed H1N1 have been under 15 years – as read off graphs in Health Protection Scotland’s recent weekly reports. For example, to 9 November 2009, Scotland had had 841 ever-hospitalizations for confirmed H1N1, 137 of whom (16 per cent) were aged 5-14 years.

Multiplying up on a population basis, England’s swine-flu deaths aged 1-14 years can be expected to be around 20. But, exactly, how many have there been? We should be told.

In August, we highlighted that England and Wales had had only 104 deaths in 1-14 year olds in 2007 that were due to diseases of the respiratory system, only 36 of which were given an ICD10 code (J10-18) for influenza & pneumonia. Pretty obviously, therefore, all deaths in children aged 1-14 years in 2009 should be ICD10 coded as a matter of priority. The task is modest because, typically, there are only 1,300 such deaths in a year in England and Wales.

Thus, will Health Protection Agency please start to monitor weekly, and publicly, England’s deaths in 2009 at ages 1-14 years against expectations based on the preceding years; and separately monitor those that are due to diseases of the respiratory system?