Congratulations to Health Protection Agency (HPA) on this week’s swine flu update, available here.

The update includes some of key data on new hospital admissions for suspected swine flu and antiviral authorizations by the National Pandemic Flu Service (NPFS), which the President of the Royal Statistical Society had wanted to have brought into the public domain.

The NPFS authorizations will have presented a methodological mire to HPA in arriving at its estimate of 30,000 new cases of swine flu for the week from Tuesday 28th July to Monday 3rd August: with uncertainty interval (of uncertain provenance) from 15,000 to 85,000. So mired, in fact, that HPA’s new note on methodology, which was promised for Friday 7 August 2009, had not been posted by 7 pm.
 
However, a problem shared is a problem halved . . . or is it doubled?
 
HPA’s Figure 5 (see Stocking Antivirals) discloses that NPFS authorizations alone for antivirals from 28 July 2009 to 3 August were at least 180,000 . . .
 
In addition, there will have been antiviral prescriptions by general practitioners for patients diagnosed as "swine-flu suspects" among those who, despite NPFS, consulted their doctor about influenza-like-illnesses. Either the decrease in new swine-flu cases has been not nearly as impressive as the headline figure of 30,000 suggests; or there is massive over-acquisition of antiviral medication.
 
HPA cannot be expected to solve the problem - unless there is put in place a sufficiently robust suite of empirical checks on UK’s estimated number and demography of those who have been H1N1v-infected. In particular, we need to know, by age-group, what proportion of the patients with an NPFS authorization actually have swine-flu.
 
Turning now to more ‘severe’ swine flu, the Table highlights the extra data on patients newly admitted to, as well as in-hospital for, suspected swine-flu. Five key messages are:
 

1. Age matters!
2. Decrease in incident hospitalizations for suspect swine-flu between weeks 30 and 31 is age-dependent: dramatic for the under 16s, notable at 17% for 16-64 year olds, but consistent with random variation in the oldest age-group.
3. New admissions naturally require staff-time for assessment and to make a treatment plan. The ratio of new hospitalizations in week 31 to inpatients was 2.3: 1 for all ages. Significant NHS staff time is needed per new admission with suspect swine-flu as well as for ongoing care
4. Finally, notice that the ratio of new hospitalizations in week 31 to inpatients is also age-dependent – it ranges from 2.9:1 in children to 1.7:1 for those aged 65+ years. The duration of hospitalization is either longer for older patients or the H1N1 epidemic of new ‘more severe’ cases is evolving differently by age-group, or both.
5. Around half of the hospitalized, and so more ‘severe’, cases of swine-flu are aged 16-64 years with around a fifth (currently) aged 65+ years. By implication, hospitalization rates for H1N1 cases increase markedly with age.