PALO ALTO, CA—This likely will come as little surprise to healthcare professionals who soldiered through it, but the 2017-18 influenza season was the most severe since VA surveillance was initiated in 2009.

VA conducts yearly surveillance for seasonal influenza, according to a presentation at ID Week 2018, primarily because the healthcare system treats a high percent of elderly patients who are at elevated risk for influenza complications, including hospitalization and death.

A study team from the VA’s Public Health Surveillance and Research office and Stanford University summarized 2017-18 national influenza activity, outbreak and vaccination data, comparing the information to previous flu seasons. VA sources were used to obtain data on hospitalizations, outpatient visits, influenza testing and antivirals from Oct. 1, 2017, to March 31, 2018.

Vaccine rates were determined from Aug. 1, 2017, and vaccination percentages were calculated based on VA users for each fiscal year. Data on outbreaks and severity were compiled from VA Issue Briefs and email surveys of VAMC infection preventionists.

Researchers ultimately presented surveillance metrics for six flu seasons are presented. They reported that, in 2017-18, high-dose vaccination increased to 20% of the total immunizations provided. Yet, outpatient visits, hospitalizations, confirmed cases and antiviral prescriptions were all more than double that of previous seasons.

In fact, 46 distinct outbreaks at 35 different VA hospitals were reported during the season.

The study found however, among 31,611 laboratory-confirmed influenza cases, veterans who had been vaccinated more than 14 days prior to LCI were significantly more likely to have an influenza-related hospitalization, than those with no documented vaccination (782, 29% vs. 5,888, 21%, p <0.01) and were less likely to have received HD vaccine compared to the overall VA patient population (375; 14% vs. 365,357; 20%, p<0.01).

Nearly 90% of those with LCI had no VA-documented vaccination this season, although some may have received vaccine outside VA, researchers explained. Overall hospitalization rate for veterans with LCI was high, 22%, and HD vaccine use increased over the seasons evaluated, but overall vaccination levels were stable.

“Vaccination did not reduce the likelihood of being hospitalized with influenza, however HD vaccine may have afforded some additional protection compared to standard dose,” the study authors concluded.

Most of what the researchers reported was in line with how the national Centers for Disease Control and Prevention viewed the last influenza season, classifying it as the first since 2003 to be high severity across all age groups.

The 2017-18 influenza season also had elevated levels of outpatient clinic and emergency department visits for influenza-like illness, high influenza-related hospitalization rates and elevated and geographically widespread influenza activity for an extended period, according to the CDC.

One factor that made the flu season so difficult was how long it lasted, the CDC reported. During the last season, ILI began to increase in November, reaching an extended period of high activity during January and February nationally, and remained elevated through the end of March. In fact, ILI peaked at 7.5%, the highest percentage since the 2009 flu pandemic, which reached 7.7%.

Because symptoms suggestive of influenza were at or above the national baseline for 19 weeks, the 2017-2018 season became one of the longest in recent years.

Worst of all, flu deaths were up in children and appeared to be so in adults. The CDC noted that, as of the end of August, 180 pediatric deaths had been reported during the 2017-2018 season, which exceeds the previously highest number of flu-associated deaths in children during a regular flu season—171 during the 2012-2013 season. Of special interest to pharmacist and others who provide immunization, most, about 80%, of the deaths occurred in children who had not received a flu vaccination this season.

Flu deaths in adults are not nationally notifiable, but, using National Center for Health Statistics Mortality Reporting System, the CDC said the percentage of deaths attributed to pneumonia and influenza was at or above the epidemic threshold for 16 consecutive weeks.
A spokesperson for the CDC told media outlets that an estimated 80,000 Americans died of flu and its complications last winter.

Hospitalization rates, meanwhile, were the highest ever recorded in the CDC’s surveillance system, breaking the previously recorded high recorded during 2014-15, when 710,000 flu hospitalizations were estimated. Most hospitalizations, 58%, were in patients 65 and older.

The overall vaccine effectiveness of the 2017-18 flu vaccine against both influenza A and B viruses was estimated to be 40%. The CDC report had a slightly different take on how vaccination affected hospitalization, however.

“While flu vaccine can vary in how well it works, flu vaccination is the best way to prevent flu and its potentially serious complications and prevents millions of flu illnesses and related doctors’ visits and tens of thousands of hospitalizations,” public health officials emphasized. “For example, during the 2016-2017 influenza season, CDC estimates that flu vaccine prevented an estimated 5.3 million illnesses, 2.6 million medical visits, and 85,000 hospitalizations associated with influenza. Similar estimates for 2017-2018 will be released in fall 2018. Influenza vaccination also has been shown to reduce the risk of flu death in children.”