 |
| Credit CDC - 2018/2019 Flu Season Severity |
#14,142
While every winter brings a flu epidemic, as the chart above illustrates, they can range wildly in severity - from the remarkably mild 2011/12 season - to the horrendous 2017/18 season which claimed at least 80,000 American lives.
Yesterday the CDC's MMWR published an update on the severity of this latest flu season; one that began relatively mild with H1N1 dominating, and became more severe as H3N2 took the lead.While a far cry from the previous year's impact, this past winter nevertheless fell into the moderately-severe category, roughly matching the 2012/13, 2014/15, and 2016/17 epidemics.
All flu seasons are unpredictable, but over the past few years the growing diversity of H3N2 viruses - and sub-optimal performance of the H3N2 component of the flu vaccine - have made flu seasons even more chaotic.
So much so, that last February - when the WHO normally decides on what strains to put in next fall's vaccine - they opted to delay their decision on the H3N2 component for 30 days (see WHO: (Partial) Recommended Composition Of 2019-2020 Northern Hemisphere Flu Vaccine).
At issue was the sudden rise of H3N2 clade 3C.3a reported in the United States (and other places), which had started last fall's season as a minor component of what appeared on track to being a relatively mild H1N1 season.By January and February, H3N2 clade 3C.3a had exploded in the United States (see CDC chart below), and in late March the WHO placed their fall flu vaccine bet on this rising strain.
Below you'll find some excerpts from a much longer MMWR report. Follow the link to read it in its entirety. I'll have a bit more when you return.
Update: Influenza Activity in the United States During the 2018–19 Season and Composition of the 2019–20 Influenza Vaccine
Weekly / June 21, 2019 / 68(24);544–551
Xiyan Xu, MD1; Lenee Blanton, MPH1; Anwar Isa Abd Elal1; Noreen Alabi, MPH1; John Barnes, PhD1; Matthew Biggerstaff, ScD1; Lynnette Brammer, MPH1; Alicia P. Budd, MPH1; Erin Burns, MA1; Charisse N. Cummings, MPH1; Shikha Garg, MD1; Rebecca Kondor, PhD1; Larisa Gubareva, PhD1; Krista Kniss, MPH1; Sankan Nyanseor, MPH1; Alissa O’Halloran, MSPH1; Melissa Rolfes, PhD1; Wendy Sessions, MPH1; Vivien G. Dugan, PhD1; Alicia M. Fry, MD1; David E. Wentworth, PhD1; James Stevens, PhD1; Daniel Jernigan, MD1 (View author affiliations) View suggested citation
Summary
What is already known about this topic?
CDC collects, compiles, and analyzes data on influenza activity and viruses in the United States.
What is added by this report?
The 2018–19 influenza season was a moderate severity season with two waves of influenza A activity of similar magnitude during the season: A(H1N1)pdm09 predominated from October 2018 to mid-February 2019, and A(H3N2) activity increased from mid-February through mid-May.
What are the implications for public health practice?
Receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its potentially severe consequences. Testing for seasonal influenza viruses and monitoring for emergence of antigenic drift variant viruses should continue year-round.
Influenza activity* in the United States during the 2018–19 season (September 30, 2018–May 18, 2019) was of moderate severity (1). Nationally, influenza-like illness (ILI)† activity began increasing in November, peaked during mid-February, and returned to below baseline in mid-April; the season lasted 21 weeks,§ making it the longest season in 10 years. Illness attributed to influenza A viruses predominated, with very little influenza B activity.
Two waves of influenza A were notable during this extended season: influenza A(H1N1)pdm09 viruses from October 2018 to mid-February 2019 and influenza A(H3N2) viruses from February through May 2019. Compared with the 2017–18 influenza season, rates of hospitalization this season were lower for adults, but were similar for children. Although influenza activity is currently below surveillance baselines, testing for seasonal influenza viruses and monitoring for novel influenza A virus infections should continue year-round. Receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its potentially severe consequences.
(BIG SNIP)
Discussion
The 2018–19 U.S. influenza season differed from recent seasons in that there were two waves of influenza A activity of similar magnitude during the season. Influenza A(H1N1)pdm09 viruses predominated overall and represented the most frequently detected influenza A virus from October 2018 to mid-February 2019; influenza A(H3N2) viruses were reported more frequently than were A(H1N1)pdm09 viruses from late February through mid-May 2019. The predominant influenza A virus also differed by geographic region and age group.
In contrast to the number of influenza A viruses reported, the number of influenza B viruses reported was low, compared with previous seasons, accounting for 4% of influenza viruses reported by public health laboratories.
The 2018–19 influenza season was longer than recent influenza seasons, and ILI activity was at or above baseline for 21 consecutive weeks. Compared with hospitalization rates during the previous five influenza seasons, the 2018–19 cumulative influenza-associated hospitalization rate (65.3 per 100,000 population) was most similar to rates observed during 2014–15 (64.1) and 2016–17 (62.0) and well below those observed during 2017–18 (102.9).(Continue . . . )
Despite the recent less-than-stellar performance by the H3N2 component of the flu vaccine, its effectiveness against H1N1 and both Influenza B viruses has been reasonably good - particularly among those under the age of 65 - the past few years.
And there's growing evidence to suggest that even a less-than-perfectly matched flu vaccine may help reduce the severity of one's illness (see CID Journal: Flu Vaccine Reduces Severe Outcomes in Hospitalized Patients).When a truly bad flu season can claim 80,000+ American lives over 3 or 4 months - as it did 18 months ago - I have to believe that some protection beats no protection at all.
While is why - in addition to practicing good fly hygiene (covering coughs, washing hands, and staying home if sick) - I'll be rolling up my sleeve again in the fall.
