The H1N1 flu strain that swept through B.C. schools and workplaces in 2009 is back with a vengeance after lurking in the background for the past few seasons.
"It's not gone away – it's been circulating at low levels over the last few years," Danuta Skowronski, a flu expert with the British Columbia Centre for Disease Control, said on Thursday.
"This year, more than 90 per cent of the viruses we have detected in British Columbia have been pandemic H1N1 virus – and that's true in other parts of North America as well – the H1N1 virus is dominating," Dr. Skowronski said.
Last year, a virus known as NH3N2 dominated.
An H1N1 outbreak in 2009 triggered a national vaccination campaign, line-ups at vaccination clinics and some classrooms being nearly emptied of children who were sick or being kept home by parents for preventive reasons.
Public-health officials do not expect a particularly deadly or dangerous flu season, Dr. Skowronski added, but are highlighting the emergence of H1N1 because it tends to hit young children the hardest, and they want to encourage people to get vaccinated before holiday socializing begins.
"I don't anticipate seeing attack rates anywhere near what they were in 2009, during that fall pandemic wave, because there is some population immunity now to keep that at bay," Dr. Skowronski said.
"We just want our population to be aware that if you think it is just the elderly who are at risk of complications of influenza, this year in particular, H1N1 tends to attack the young more than the very old."
That caution applies particularly to children under the age of five, who were not around during the 2009 pandemic and may not have been exposed to even the low levels of H1N1 that have circulated in recent years.
Most people infected by H1N1 will recover, but some will require hospital care, and the illness can be fatal, with the biggest risk to young children, especially those with chronic health conditions.
Testing over the summer in the Lower Mainland found the lowest immunity rates to H1N1 among the under-five age group.
But tests also showed relatively low levels of immunity among adults aged 20 to 60.
Those results and other data led the BCCDC to make a new immunization push.
"We are starting to see an increase in pandemic H1N1 in our communities – it is not peaking yet, but you don't want to get immunized at the peak of the epidemic. It takes a couple of weeks to mount that antibody response.
"You want to get the maximum benefit – play your cards right, get immunized now, you'll be protected during the peak."
This year, the B.C. government implemented a policy that requires health-care workers to get a flu shot or wear a mask on the job. Some objected, but provincial health officials said it was necessary with a vaccination rate of under 50 per cent for health-care workers.
In its most recent influenza bulletin, issued on Thursday for the two-week period ended Dec. 14, the BCCDC reported "increasing but still low-level influenza activity" in the province.
Medical Services Plan (MSP) claims for influenza illness remain within the expected range, but the percentage of swabs testing positive for flu is increasing – from 4.5 per cent in late November to 18.2 per cent in mid-December – and most of those positive tests are H1N1.
That trend is expected to continue as the holiday season moves into full swing. The very things people will be doing – mingling of young and old, hugs and kisses – are great for spreading the flu.
"People are going to be congregating. … You have age bands mixing, and that's going to facilitate the spread."