Incidence of flu a long way off epidemic levels

Q: Was half the country really sick this Christmas – or did it just seem that way?

With people back at work after the holiday season, there is an impression of more than the usual number of families whose Christmas was blighted by illness. So was half the country sick this Christmas or did it just seem that way?

Infectious diseases such as influenza and other viruses peak during the winter. The exact peak differs from year to year but Christmas and the new year occasionally coincide with the highest circulation of winter bugs.

The Health Protection Surveillance Centre’s latest report shows an increase in recent weeks in influenza-like illness (ILI) during the last fortnight of 2012.

This is measured by reports from a nationwide network of “sentinel” general practices, which report each week on the number of consultations involving patients with flu-like symptoms. The ILI rate is now above Ireland’s baseline rate of 21 per 100,000 population, although nowhere near epidemic levels.

When samples from these patients are analysed by the National Virus Reference Laboratory, many turn out to be other respiratory viruses such as parainfluenza and the respiratory syncytial virus.

These cause flu-like symptoms but generally do not result in the absolute prostration often seen with full-blown flu.

The reference lab has however identified some influenza A and influenza B virus circulating in Ireland. The indication from across Europe is that influenza viruses are on the march.

The other major infectious disease at this time of year is the norovirus, also known as the winter vomiting bug. It causes a nasty 48 hours of nausea, vomiting and sometimes diarrhoea that leaves patients feeling washed-out.

This bug is most definitely on the rise. The number of cases of the virus reported to the HPSC represents just the tip of the iceberg; even so there has been a significant increase in the number of reported cases of norovirus for 2012 compared with 2011.

This week the European Centre for Disease Control reported that a new strain of the virus was becoming more prevalent. The GII.4 variant has been picked up by labs in Australia and the US, with the suggestion that it causes a more severe bout of illness because people have yet to develop an immunity.

Anecdotal reports of additional Christmas “sickness” this winter are quite possibly due to the vomiting bug but unlikely to have been caused by the influenza virus.