Flu Vaccine Cuts Adult Deaths and Child Hospitalizations

Marcia Frellick

October 07, 2019

WASHINGTON, DC — Influenza vaccination reduces the risk for severe outcomes in adults by more than one-third, and the risk for hospitalization in children by half, according to the result of two new studies.

The size of these studies over multiple seasons and the rigor of the investigations should give physicians powerful new evidence for vaccine-hesitant patients and parents, said Kristina Bryant, MD, from Norton Children's Hospital and the University of Louisville in Kentucky.

"When our patients don't accept flu vaccine, we need to ask why," she told Medscape Medical News. A common answer is that they had the vaccine one year and still got the flu. These studies provide evidence that although the vaccine might not always prevent illness, it likely will reduce severity and even prevent deaths, she said.

Adult Investigation Spanned Five Seasons

"Our study provides more evidence that getting a flu vaccine reduces the severity of influenza illness," said Shikha Garg, MD, a medical epidemiologist from the Centers for Disease Control and Prevention (CDC), who was lead researcher on the adult study.

"And we showed this across five flu seasons and across a wide geographic area," she told attendees here at IDWeek 2019.

Garg and her colleagues assessed data on 43,608 patients, at least 18 years of age, hospitalized at some point over the course of five flu seasons — 2013/14 to 2017/18 — from the Influenza Hospitalization Surveillance Network (FluSurv-NET), which contains information from more than 250 acute care hospitals in 13 states.

They determined vaccination status using medical records, vaccine registries, and interviews.

The researchers found that the risk for severe outcomes was 36% lower in patients who had been vaccinated than in those who had not.

Specifically, for adults 18 to 64 years who were hospitalized with H1N1, vaccination reduced the risk for death by 36%, the risk for admission to the intensive care unit (ICU) by 19%, the risk for mechanical ventilation by 34%, and the risk for pneumonia by 17%.

In addition, ICU stays were shorter for patients who had been vaccinated.

For patients 65 years and older, vaccination reduced the risk for ICU admission by 28% and the risk for mechanical ventilation by 46%.

For patients with the H3N2 virus or the B virus, there was no significant reduction in severe outcomes with vaccination.

"Fewer data are available, particularly in recent years, on how well the vaccine protects against severe disease in children in the United States," said Angela Campbell, MD, a medical officer in the Influenza Epidemiology and Prevention Branch of the CDC, who presented findings from a study of the effectiveness of the vaccine in children.

Vaccinated Children Half as Likely to Be Hospitalized

Every season, regardless of which viruses are most common and how severe a season it is, children are hospitalized with the flu, Campbell pointed out. But "most recent national estimates of influenza vaccine effectiveness have measured protection against outpatient medical visits."

In their study, Campbell and her colleagues conducted an unusual analysis of vaccine protection in the most severe cases.

They compared the number of flu vaccinations in children with laboratory-confirmed influenza with the number in children who had not contracted influenza, and applied mathematical models adjusted for season, time of admission in relation to treatment, symptom onset, and season peak.

Their cohort consisted of 3630 children 6 months to 17 years of age who were hospitalized with acute respiratory illness at one of seven pediatric medical centers over the course of two influenza seasons.

Children who were vaccinated were half as likely to be hospitalized with the flu as those who were not (49% in the 2016/17 season and 51% in 2017/18 season).

"Protection was 33% for H3N2 viruses, 76% for H1N1 viruses, and 59% for B viruses when we combined the two seasons," Campbell reported.

"About 60% of the children hospitalized had an underlying medical condition. That means 40% were otherwise previously healthy children," she added.

The CDC recommends annual flu vaccination for everyone 6 months of age and older, but reports that only 45% of adults and 63% of children were vaccinated in the 2018/19 season.

Garg, Campbell, and Bryant have disclosed no relevant financial relationships.

IDWeek 2019: Abstracts 898 and 899. Presented October 3, 2019.

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