New Zealand swine flu data shows the virus targeted younger people and dominated other virus strains after circulating for just one month, American health officials say.
The US Centres for Disease Control and Prevention (CDC) in Atlanta has been watching the spread of the virus during the New Zealand winter to gain ideas of how it will behave in the coming northern hemisphere flu season.
Centre staff are planning a vaccination campaign focusing on people disproportionately affected by the A (H1N1) virus -- w children, pregnant women, and adults with underlying health conditions.
The CDC said in a study published online in a weekly report that New Zealand entered the southern flu season with both seasonal and 2009 pandemic influenza strains circulating.
New Zealand was the first overseas country to report importation of the swine flu.
The number of viruses identified as 2009 pandemic influenza rapidly overtook the number identified as seasonal influenza, said the lead author, Environmental Science and Research virologist Sue Huang, head of the World Health Organisation national influenza centre in Upper Hutt.
For the 2009 influenza season, 95 sentinel GPs were recruited, representing all of the country's 24 health districts and with a combined patient population of 409,044, approximately 9.6 percent of the population.
During the study period -- the weeks ending May 3 through August 2 -- a total of 6280 consultations for flu symptoms were reported from the 24 health districts.
The percentage of viruses identified as swine flu increased from 14 percent during the week of June 8-14 to 80 percent during the week of June 29--July 5.
The highest concentration of cases occurred in South Auckland, where people went to the doctor with flu symptoms at a rate of 1308 per 100,000 in a single week, from July 6 to July 12. A weekly rate of 100 to 149 would normally be considered "moderate" while rates of more than 400 are regarded as an epidemic.
Consultation rates varied greatly among health districts those with the next highest consultation rates were Wellington (709) and South Canterbury (505).
Compared with the same period in 2008 and 2007, the national 2009 weekly consultation rate first shot up in the second week of June and increased sharply from the week ending June 21 to the week ending July 12.
The consultation rate peaked at 287 consultations per 100,000 patient population in the week ending July 19, approximately three times the peak rate of 95 consultations recorded in 2008.
The highest consultation rates were recorded among children aged 1-4 years followed by infants aged 1 year and young people aged 5-19, 20-34, 35-49 years, 50-64 yearsand over 65.
A total of 1963 swabs were sent to virology laboratories from sentinel GPs during the study period, compared with 543 swabs in 2008, and 527 influenza viruses were identified.
The predominant strain was 2009 pandemic influenza A (H1N1) (63 percent), followed by seasonal influenza A (14 percent) seasonal influenza A (H1N1) (13 percent), influenza A not subtyped (8 percent), seasonal influenza A (H3N2) (8 swabs), and influenza B not typed (1 swab).
A network of hospital laboratories reported a total of 3931 influenza viruses, again with the swine flu dominating as its percentage rose 22 percent during June 8 -- 14 to 66 percent during June 29 -- July 5.
