This article is more than 5 years old. Information may no longer be current.
CDC: More signs of a severe flu season in US
Click Here to Manage Email Alerts
.png)
There are more signs this week that the United States is experiencing a particularly severe influenza season.
In the week ending Jan. 27, CDC officials said there were 51.4 hospitalizations due to influenza per 100,000 people in the U.S. If this rate is maintained through the end of the season, it would be the highest rate of influenza-related hospitalizations since the CDC changed the way it tracks them in 2010, when it started using representative surveillance data from around a dozen sites in the country.
For a comparison, the hospitalization rate in 2014-2015 — a recent benchmark for bad influenza seasons — was 43.5 per 100,000 people. The CDC estimates that influenza hospitalized 710,000 people that season. Officials said this season’s total could exceed that amount.
“This season is a somber reminder of why flu is one of the world’s greatest public health challenges,” said Anne Schuchat, MD, who took over as acting CDC director when Brenda Fitzgerald, MD, resigned this week. “Unfortunately, our latest tracking data indicate that flu activity is still high and widespread across most of the nation and increasing overall.”
H3N2, a vaccine-dodging influenza A strain known for causing relatively severe illness, remains the dominant circulating strain, as it has been all season. Adults aged 65 years and older continue to be the hardest-hit population, followed by adults aged between 50 and 64 years and children aged younger than age 5 years, according to Daniel B. Jernigan, MD, MPH, director of the CDC’s Influenza Division.
The CDC reported 16 more influenza-related pediatric deaths, raising the total to 53 this season. Since they became nationally notifiable in 2004, there have been as many as 171 pediatric deaths from seasonal influenza. During the 2009 pandemic, there were 358.
Jernigan said adult mortality due to pneumonia or influenza — which is based on death certificates — increased from 9.1% to 9.7% over the past week and could surpass the recent peaks of 11.1% and 10.8% seen in the 2012-2013 and 2014-2015 seasons, respectively.
“This season is turning out to be a particularly challenging one,” he said. “It has been compounded by lots of flu occurring nationwide simultaneously over several weeks. This is an unusual pattern in the U.S.”
Influenza activity was elevated for the 10th consecutive week and actually increased, Schuchat said. The proportion of outpatient visits due to influenza-like illness increased again, from 6.6% to 7.1%. Only two seasons in the last 15 years saw higher rates of this important surveillance statistic: the 2009 H1N1 pandemic (7.7%) and 2003-2004 H3N2-dominated season (7.6%). States experiencing high influenza activity increased from 39 to 42 in a week, Jernigan said.
According to the most recent CDC data, 17.5% of the 584,362 specimens tested in clinical labs since Oct. 1 were positive for influenza, including 81.3% that tested positive for influenza A. Among influenza A viruses that were subtyped in public health labs, 89% were H3N2 viruses.
Jernigan said there is no laboratory evidence yet that circulating H3N2 viruses have drifted significantly this season. However, H3N2 viruses prepared using egg-based vaccine manufacturing methods already have adaptations that make them less similar to circulating viruses, and the strain is notorious not only for causing relatively severe illness, but for its ability to evade the seasonal vaccine.
The CDC will not have vaccine effectiveness estimates for the U.S. until mid-February, but there are signs that this season’s H3N2 component — perhaps unsurprisingly — has not been protective.
Researchers this week published interim estimates of the vaccine’s effectiveness in Canada this season, including an estimate that it has been just 17% effective against H3N2. That is marginally better than Australia, which reported that the vaccine was 10% effective against H3N2 during the Southern Hemisphere’s influenza season. But it is lower than the vaccine’s effectiveness against the strain in the U.S. last year, which was around 32%.
CDC officials had said that last year’s vaccine performance against H3N2 in the U.S. would be a better barometer of how this year’s formulation would work than what happened in Australia. The data from Canada has surprised some, including Schuchat, who said she expects the U.S. numbers will be close to Canada’s.
“This is consistent with our concerns that the H3N2 influenza vaccine effectiveness is lower than against other types and that in recent years it’s been challenging to even get H3N2 protection,” she said. “We continue to recommend the flu vaccine, even though we know most flu vaccines have low effectiveness against H3N2 viruses. Effectiveness against other flu viruses is better. The vaccine may also reduce the symptoms if you get the flu.”
In a bit of good news, influenza appears to be loosening its grip in the West, where Schuchat said there were signs for a second straight week that activity was easing up. Oregon was the only state in the continental U.S. where influenza activity went down and was not considered widespread. Jernigan said influenza activity was high in the East and South.
“We are by no means out of the woods,” Schuchat said. “Most seasons last up to 20 weeks and we’ve likely got several weeks left of increased activity.” – by Gerard Gallagher
References:
CDC. FluView. https://www.cdc.gov/flu/weekly/index.htm. Accessed February 2, 2018.
Skowronski DM, et al. Euro Surveill. 2018;doi:10.2807/1560-7917.ES.2018.23.5.18-00035.
Disclosures: Jernigan and Schuchat work for the CDC.