MMWR: Reviewing Our High Severity 2017-2018 Flu Season

#13,354


By just about any metric you'd care to apply, last winter's U.S. flu season was the roughest, and most prolonged, in recent memory. Influenza surveillance methods have changed over the years, but reasonably accurate comparisons can be made going back to 2004.

Lasting 19 weeks, at its peak the number of doctor's visits for ILI (influenza-like illness) matched the levels seen during the 2009 pandemic, and the number of hospitalizations exceeded that pandemic year. 

Data is still being collected and analyzed, but the number of flu-related pediatric deaths equals (n=171) any year (with the exception of the 2009 pandemic) since reporting of these cases became mandatory in 2004.

It would not be surprising to see that number climb higher in the weeks ahead as reports are often delayed.

Yesterday's MMWR carried a lengthy review of last winter's flu season, and looks ahead at the composition of next fall's flu vaccine. 

First, the summary:

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 2017–18 influenza season was a high severity, A(H3N2)-predominant season. Influenza activity indicators were notable for the volume and intensity of influenza cases that occurred in most of the country at the same time. Record hospitalization rates and high numbers of influenza-associated pediatric deaths also were reported.

What are the implications for public health practice?
Receiving a seasonal flu 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 novel influenza A virus infections should continue year-round.

 

The full report, which will require a commitment of about a half hour to read, delves deeply into the antigenic characterization of the flu viruses collected, the Vaccine Effectiveness (VE) of both cell-based and egg-based vaccines against them, the (low) level of anti-viral resistance markers detected, and the impact of last year's flu season on different demographics.

I've included a couple of small excerpts, but you'll want to follow the link to read the entire report.   I'll return with a postscript.

Update: Influenza Activity in the United States During the 2017–18 Season and Composition of the 2018–19 Influenza Vaccine

Weekly / June 8, 2018 / 67(22);634–642

33-41 minutes

(Excerpts)

Severity Assessment

In 2017, CDC began using a new methodology to classify influenza season severity using three indicators: 1) the percentage of visits to outpatient clinics for ILI from ILINet, 2) the rates of influenza-associated hospitalizations from FluSurv-Net, and 3) the percentage of deaths resulting from pneumonia or influenza from NCHS (8). This approach uses data from past influenza season indicators to calculate three intensity thresholds (ITs) (additional information is available at https://www.cdc.gov/flu/professionals/classifies-flu-severity.htm).

These ITs help assess the historic chance that surveillance system data will exceed a certain threshold. CDC then classifies the severity of the current influenza season by determining which IT was crossed by at least two of the peak values from the above indicators. Based on this method, the severity of the 2017–18 season was classified as high severity overall and high severity for each age group (children and adolescents, adults, and older adults). This is the first time that each age group was classified as high in the same season, in a retrospective analysis going back to the 2003–04 season (Figure 3).

 (SNIP)

 Discussion

Previous influenza A(H3N2)-predominant seasons have also been associated with increased hospitalizations and deaths; however, the 2017–18 season followed an A(H3N2)-predominant season, and all severity indicators were higher than during the 2016–17 season. The majority of A(H3N2) viruses were genetically characterized as 3C.2a clade, similar, but genetically distinct from the 3C.2a1 subclade that predominated during the 2016–17 season, and from the viruses that circulated during Australia’s 2017 influenza season (7,10). Outside the United States and Canada, A(H3N2) viruses did not predominate in other Northern hemisphere temperate countries. Further studies are needed to understand the virologic, host, or environmental factors responsible for this high severity season.

The severity of this influenza season highlights the importance of public health measures to control and prevent influenza. Annual influenza vaccination remains the most effective way to prevent influenza illness. Although influenza activity in the United States is typically low during the summer, influenza cases and outbreaks can occur, and clinicians should consider influenza in the differential diagnosis of respiratory illnesses at any time of year. CDC recommends prompt treatment with influenza antiviral medications for persons with confirmed or suspected influenza who are severely ill or at high risk for serious influenza complications. 

Health care providers should consider novel influenza virus infections in persons with ILI and swine or poultry exposure, or with severe acute respiratory infection after travel to areas where avian influenza viruses have been detected. Providers should alert the local public health department if novel influenza virus infection is suspected.

Clinical laboratories using a commercially available influenza diagnostic assay that includes influenza A virus subtype determination should contact their state public health laboratory to facilitate transport and additional testing of any unsubtypeable influenza A–positive specimen. Public health laboratories should immediately send unsubtypeable influenza A viruses to CDC, because early identification and investigation are critical to ensuring timely risk assessment and implementation of appropriate public health measures.

(Continue . . . )

As we discussed last weekend in The `Other' Novel Flu Threat We'll Be Watching This Summer, 2017 saw a surge in swine variant flu infections in the United States (n=67), primarily associated with visits to agricultural exhibits in county and state fairs. 

This was the second highest yearly total reported since surveillance began in 2005, only surpassed by 2012.

While this isn't the first time we've seen a reminder from the CDC to health care providers to `consider novel influenza virus infections in persons with ILI and swine or poultry exposure', the warnings do appear to have become more prominent in the past year or two.

The big lesson from last year's flu season is we don't have to have a pandemic, or novel flu outbreak, to suffer a high impact severe flu season.

While the flu vaccine turned in another disappointing performance against H3N2 last winter, it did pretty well against H1N1 and influenza B. 

And the more we learn about the complications of influenza (see Eur. Resp.J.: Influenza & Pneumonia Infections Increase Risk Of Heart Attack and Stroke), the more worthwhile that protection becomes.

Which is why I'll get the shot again this year, and diligently practice good `flu hygiene' (ie. cover coughs, use hand sanitizer, stay home when sick, etc.), and  I'll urge others to do the same.  

Because - with something as serious as a heart attack - some protection, beats no protection at all.