Review of severe 2017-18 flu season shows drop in vaccination

Wrapping up its assessments of last flu season, the US Centers for Disease Control and Prevention (CDC) said flu was severe, with unusually high levels of illnesses, hospitalizations, and deaths—making for the worst season since the 2009 H1N1 pandemic.

Also, the CDC said in a separate update that an annual survey suggests that flu vaccination coverage declined last season in adults.

Meanwhile, with the nation just a few weeks into the 2018-19 season, flu levels are low, with all three strains circulating, according to the CDC's latest weekly FluView report today.

Most severe non-pandemic season

Last year, flu activity started its rise in November and remained at high levels for several weeks into the first part of 2018. H3N2 was the predominant strain through February, with influenza B becoming more common in March, which isn't unusual for the latter part of the flu season.

Based on its modeling estimates, flu sickened 48.8 million people and resulted in 22.7 million clinic visits, 959,000 hospitalizations, and 79,400 deaths.

For comparison, the CDC estimates that 60 million people were sickened during the 2009 H1N1 pandemic, and 2017-18 outpaced all other recent non-pandemic flu seasons in severity.

The CDC saw high levels of flu hospitalization in the very young and very old, a pattern it typically sees with flu. It said, however, that hospitalization rates for all age-groups were the highest it has seen since 2005, when it began including all age-groups in hospital flu surveillance.

"Our estimates of hospitalizations and mortality associated with the 2017–2018 influenza season continue to demonstrate how severe influenza virus infection can be," the CDC said. Older adults accounted for 70% of hospitalizations and 90% of deaths, underscoring how seniors are especially vulnerable to severe flu.

In working-age adults, a group that typically has the lowest flu vaccine uptake, flu caused an estimated 10,300 deaths.

As last year's flu season wound down, the CDC said reports of child flu deaths were the highest for a nonpandemic year since the agency started tracking them in 2004. What's more, the agency said the 183 reports it received for 2017-18 is probably an underestimate, because not all children whose deaths are related to flu were tested.

Based on hospitalization rates and death certificate causes of death in patients who were and weren't hospitalized, the CDC estimates there were in reality more than 600 pediatric flu deaths.

Decline in adult flu vaccine uptake

An ongoing telephone survey that the CDC uses to assess a range of health conditions and risk behaviors in adults suggests that flu vaccine coverage in that age-goup last season was 37.1%, reflecting a 6.2-percentage-point drop from the previous season and the lowest level in the past seven seasons.

However, the CDC urged caution in interpreting the results, given the self-reporting of vaccination status and the fact that its early estimates from other data sources did not show a drop in uptake.

Its estimate from the Behavioral Risk Factor Surveillance System (BRFSS) survey suggested that although vaccine coverage varied by age and state, levels declined in all age-groups and most states. Coverage varied widely by state, from 29.2% in Louisiana to 46.3% in West Virginia. Levels declined in 37 states compared with the previous season and held steady in 13 states and the District of Columbia. When researchers looked at changes by ethnic group, flu vaccine coverage declined for all except American Indian/Alaska Natives.

Despite limitations of the data sources it uses to estimate flu vaccine uptake in adults, the main message is still that coverage remains low in this age-group, with only about 4 in 10 reporting vaccination.

"As the 2018–19 season is underway, it is important that providers prioritize flu vaccination for their patients," the CDC said. "This includes client reminders when flu vaccine supplies become available, assessing the vaccination status at every visit, making an effective recommendation for vaccination, and offering the vaccine."

Current activity still low

In its weekly report covering flu activity through last week, the CDC said disease activity is still low nationally, with only four states reporting local flu activity: Massachusetts, New Hampshire, North Dakota, and Oregon.

All three strains are circulating, though 2009 H1N1 is the most commonly identified virus.

Clinic visits for flulike illness last week were at 1.5%, remaining well below the national baseline of 2.2%. All of the CDC's regions were below their specific baselines.

The percentage of deaths from pneumonia and flu was 5.3%, below the epidemic threshold of 5.9%. No new pediatric flu deaths were reported, keeping the total at one so far.

At clinical labs, the percentage of respiratory samples that tested positive for flu was 0.6%.

The CDC recommends that everyone age 6 months and older be vaccinated against flu by the end of October.

See also:

Oct 25 CDC estimate of 2017-18 flu burden

Oct 25 CDC estimates of 2017-18 adult flu vaccine coverage

Oct 26 CDC FluView report

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