The 2017–18 Flu Season: High Severity for Every Age Group, and the 2018–19 Vaccine

The 2017–18 U.S. influenza season (October 1, 2017–May 19, 2018) was a high-severity season: many outpatient and emergency visits for influenza-like illness (ILI), high hospitalization rates, and flu widespread across the country for a long stretch. ILI began rising in November, stayed very high through January–February and remained elevated into March. Influenza A(H3N2) dominated through February and for the season overall; influenza B took over from March.

The viruses

• Clinical lab positivity peaked for five straight weeks (January 13–February 10) at 26.1%–26.9%.
• Of subtyped influenza A, 84.9% was A(H3N2) and 15.1% A(H1N1)pdm09; of influenza B with lineage known, 88.8% was B/Yamagata and 11.2% B/Victoria.
• By age, 34.3% of positive patients were 65 or older; A(H3N2) ranged from 51.2% of viruses in ages 5–24 to 70.0% in those 65 and older.

How well they matched the vaccine

• A(H1N1)pdm09: all in subclade 6B.1; 99.9% well inhibited by antisera to the vaccine reference virus.
• A(H3N2): mostly clade 3C.2a. 93.4% were well inhibited by antisera to the cell-grown reference virus — but only 48.2% by antisera to the egg-grown vaccine virus, since egg adaptation can change a virus. 77.3% were well inhibited by antisera to the egg-grown A/Singapore virus chosen for the next vaccines.
• B/Yamagata: all clade Y3 and antigenically similar to the vaccine virus.
• B/Victoria: few, but 81.3% carried a six-nucleotide deletion (the V1A.1 group), and only 19.6% resembled the vaccine virus — clear antigenic drift.

Antivirals: of 4,619 viruses tested, only 11 A(H1N1)pdm09 viruses (1.0%) resisted oseltamivir and peramivir; none resisted zanamivir, and all A(H3N2) and B viruses were susceptible. Resistance to the adamantanes remains high, and they are not recommended.

How bad it was

• Outpatient ILI: at or above the 2.2% baseline for 19 straight weeks, above 7.0% for three weeks, peaking at 7.5% in the week ending February 3 — the third highest since 1997–98. At the peak, 46 of 53 jurisdictions (87%) had high ILI activity.
• Geographic spread: 50 jurisdictions (93%) reported widespread flu for three weeks in January.
• Deaths from pneumonia and influenza: above the epidemic threshold for 16 straight weeks, peaking at 10.8% of deaths — the highest since 2014–15.

Percentage of outpatient visits for influenza-like illness by week. Image from the Centers for Disease Control and Prevention

Hospitalizations: in a surveillance network covering about 27 million people, 30,453 laboratory-confirmed flu hospitalizations — 106.6 per 100,000, peaking in the week ending January 6. Adults 65 and older had by far the highest rate and made up about 58% of cases.

• The overall and adult rates were the highest recorded since adults were added in 2005–06; children's rates topped earlier high-severity A(H3N2) seasons, though not the 2009 pandemic.
• 92.4% of hospitalized adults had a high-risk condition — most often cardiovascular disease, metabolic disorders, obesity or chronic lung disease; 56.7% of hospitalized children had one, most often asthma. 30.7% of hospitalized women aged 15–44 with known status were pregnant.

Cumulative flu hospitalization rates by season. Image from the Centers for Disease Control and Prevention

Severity: under a method CDC began using in 2017, the season was high severity overall and for every age group — the first time all groups were rated high in the same season, looking back to 2003–04.

Season severity by age group. Image from the Centers for Disease Control and Prevention

Children's deaths: 171 — equal to the 2012–13 record outside the 2009 pandemic. Mean age 7.1 years; 57% died after hospital admission; about half had no high-risk condition. Only 22% of vaccine-eligible children who died had been vaccinated.

An unusual pattern: hospitalizations and deaths peaked before lab positivity and outpatient visits, and older adults peaked before children and young adults — the reverse of the usual order.

The vaccine

• 2017–18 effectiveness (interim, February 2018): 36% overall — 25% against A(H3N2), 67% against A(H1N1)pdm09, 42% against influenza B. Even less effective vaccines prevent much severe illness: in 2016–17, vaccination averted an estimated 5.29 million illnesses, 2.64 million medical visits and 84,700 hospitalizations.
• 2018–19 composition: an A/Michigan/45/2015 A(H1N1)pdm09-like virus, an A/Singapore/INFIMH-16-0019/2016 A(H3N2)-like virus and a B/Colorado/06/2017-like (B/Victoria) virus; quadrivalent vaccines add B/Phuket/3073/2013-like (B/Yamagata). The B/Victoria change answers the spreading drifted V1A.1 virus; the A(H3N2) change was made because the egg-grown A/Singapore virus better matches circulating viruses — not because of drift.

What to do

• Get vaccinated every year — still the best protection.
• Treat promptly with antivirals anyone with confirmed or suspected flu who is severely ill or at high risk.
• Think flu year-round, and consider novel influenza in people with ILI after swine or poultry contact or severe respiratory illness after travel where avian influenza circulates — alerting the health department and sending unsubtypeable influenza A specimens on for testing.

Sources

Based on "Update: Influenza Activity in the United States During the 2017–18 Season and Composition of the 2018–19 Influenza Vaccine," by Rebecca Garten, Lenee Blanton and colleagues, Morbidity and Mortality Weekly Report 67(22), Centers for Disease Control and Prevention; a work of the United States government in the public domain.