Flu activity is heating up in Clark County and across the country, and health officials say the strain circulating doesn’t typically respond as well to the annual flu shot as other strains.
While flu activity was low in October, virus circulation across the U.S. has been increasing since the beginning of November. And several influenza indicators are higher than typical for this time of the year, according to the Centers for Disease Control and Prevention.
Influenza A strains are the most common flu circulating this season, with the H3N2 strain predominant. While the circulating strains appear to be similar to those used to build the seasonal vaccine, that vaccine has historically been about 30 to 40 percent effective against the H3N2 strain, according to the CDC.
In the last decade, overall vaccine effectiveness has been between 40 and 60 percent, with the exception of the 2014-15 flu season, when it was 19 percent effective.
The lower effectiveness of the vaccine against H3N2 is attributed to genetic changes in the virus as it circulates, as well as changes to the virus as it’s grown in eggs, which is part of the production process for vaccines. While those changes happen to all influenza viruses, H3N2 appears particularly susceptible to changes that make it less compatible with the vaccine and, therefore, less effective, according to the CDC.
“Even with influenza vaccine effectiveness in the range of 30 percent to 60 percent, influenza vaccination prevents millions of infections and medical visits and tens of thousands of influenza-associated hospitalizations each year in the United States,” according to a CDC report issued Friday.
The H3N2 strain is also typically associated with greater rates of hospitalizations and flu-related deaths, according to the CDC.
Flu activity rises
In Clark County, flu activity picked up in mid-November.
From Nov. 12 to 18, nearly 11 percent of influenza tests came back positive. A positivity rate of more than 10 percent indicates flu transmission is occurring in the community. Since mid-November, the positivity rate in Clark County has hovered just below that threshold.
Clark County Public Health has received one report of influenza-like illness in a care facility this flu season, but it wasn’t confirmed via laboratory testing as an outbreak. No flu deaths have been reported yet this flu season in Clark County.
Last flu season, Public Health recorded 25 flu-associated deaths, nearly all of which occurred in elderly people or people with other underlying health conditions.
