Influenza activity has been ticking up as the 2017-18 flu season progresses. Twenty pediatric influenza deaths have occurred since the season began in October 2017. The Centers for Disease Control and Prevention’s most recent report for the week ending January 6 shows that 26 states are reporting a high level of influenza-like illness, and 12 states report a moderate level of activity.
This influenza season is more severe and active than last season, and it closely resembles the 2014-15 season, which is regarded as a severe season. At this point in that season, 19 pediatric influenza deaths had occurred. For another point of comparison, the proportion of outpatient visits attributed to influenza-like illness (ILI) this week was 5.8%; in the same week of 2014-15, it was 4.4%. The CDC estimates that the 2014-15 season resulted in about 34 million cases of influenza and about 20,000 deaths related directly to influenza (data on associated respiratory and circulatory deaths don’t seem to be available yet for that season, but in recent previous seasons those influenza-associated deaths number an additional 12,000 to 39,000).
Recent new stories have covered influenza deaths in previously healthy individuals, such as a 40-year-old woman who ran marathons, a 21-year-old personal trainer, and a 10-year old boy who played hockey. While most influenza-related deaths occur in the very old and very young, every year previously healthy people not in risk groups get the flu and die. The highly publicized deaths this year do not seem to be out of the ordinary, especially in a year dominated by high flu activity from the H3N2 influenza A strain. H3N2 is known to cause more severe illness than some other flu strains.
Regarding vaccine effectiveness (VE)*, influenza vaccines contain antigens of three or four influenza strains, and each component may have a different VE. VE for H3N2 is typically lower than for other influenza types, often in the 30%-40% range. However, in Australia’s recent influenza season, which often mirrors what the following US influenza season will be like, the H3N2 VE was only 10%. Experts note that while the H3N2 reference strain used to produce Australian vaccines was genetically similar to circulating virus strains, changes to the H3N2 component during production in chicken eggs made it less effective against circulating viruses.
To some degree, this problem seems to apply to US vaccines and circulating viruses here. As of December 8, 2017, the CDC reported:
The majority of influenza viruses collected in the United States since October 1, 2017, were characterized antigenically or genetically as being similar to the cell-grown reference viruses representing the 2017–18 Northern Hemisphere influenza vaccine viruses, indicating that significant antigenic drift has not occurred at this time. However, some currently circulating A(H3N2) viruses are less similar to egg-adapted viruses used for production of the majority of U.S. influenza vaccines.
Experts caution that the low VE seen in Australia last season doesn’t necessarily mean US VE will be so low. Some predict that analyses at the end of the flu season will demonstrate a VE for H3N2 in the 30% range. Combined VE will likely be higher.
By early November 2017, the CDC estimated that about 39% of Americans had been vaccinated for influenza this season. This estimate will likely increase as the season progresses; coverage has ranged from 38.8%-43.3% in adults in recent influenza seasons.
*VE is a point estimate that represents the reduction in risk provided by receipt of flu vaccine. The official VE estimate is determined when CDC compares the odds of vaccination among outpatients with acute respiratory illness and laboratory-confirmed influenza infection to the odds of vaccination among outpatients with acute respiratory illness who test negative for influenza infection. So, a VE of 60% means that the flu vaccine reduces a person’s risk of the outcome (outpatient visit due to lab-confirmed influenza) by 60%.