The 2018–19 Flu Season at Midseason

This page describes a midseason CDC report on the 2018–19 influenza season, with data through February 2, 2019, and the recommendations of that time.

CDC gathers data on flu activity and viruses across the United States from laboratories, outpatient clinics, hospitals and death records. From September 30, 2018, to February 2, 2019, flu activity was low in October and November, rose in late December and stayed elevated into February. Compared with recent seasons, this was a low-severity season so far, with fewer outpatient flu-like illness visits, lower hospitalization rates and fewer pneumonia and influenza deaths.

Which viruses

• Clinical labs tested 536,301 specimens; 10.1% were positive, 95.7% of those influenza A. The weekly positive share ranged from 1.7% to 21.6%.
• Public health labs found 12,200 positives among 30,344 specimens, 97.2% influenza A. Of influenza A viruses subtyped, 80.0% were A(H1N1)pdm09 and 20.0% A(H3N2). Influenza B was under 3%.
• H1N1 predominated nationally and in every age group, but H3N2 predominated in the Southeast and made up a growing share elsewhere — among people aged 5–24, from 24.4% to 46.2% between late December and early February.

Influenza-positive tests reported by clinical laboratories. CDC figure.

How well the vaccine matched

• Nearly all H1N1 viruses were in clade 6B.1, and 98.5% of those tested matched the vaccine reference virus, though genetic diversity within the clade was growing.
• H3N2 was more varied. Of 145 viruses tested, 70.3% were well inhibited by antiserum against the cell-grown vaccine reference virus, but 29.7% reacted poorly — nearly all in clade 3C.3a, which grew from 16% of sequenced H3N2 viruses in November to 51% in December and spread from the Southeast across the country. Only 28 of the 145 matched the egg-grown vaccine virus, probably because of changes from growing it in eggs.
• All B/Yamagata viruses tested matched their vaccine component; 71.4% of B/Victoria viruses did.

Antiviral resistance

Of 823 viruses tested, more than 99% were susceptible to oseltamivir and peramivir and all to zanamivir; four H1N1 viruses showed reduced or highly reduced inhibition by oseltamivir. Resistance to the older adamantane drugs remained high among influenza A viruses.

Illness, hospitalization and deaths

• Outpatient visits for flu-like illness had been at or above the national baseline of 2.2% for 9 weeks, reaching 4.3% in the week ending February 2 — with all 10 HHS regions above their baselines. Past seasons' peaks ranged from 3.6% (2015–16) to 7.5% (2017–18).
• Widespread activity was reported by 48 jurisdictions (89%) in the week ending February 2.
• Hospitalizations (FluSurv-NET, about 9% of the population): 5,791 lab-confirmed, a cumulative rate of 20.1 per 100,000, highest among adults 65 and older (53.0) and children under 5 (33.5). Among hospitalized adults with information, 90.2% had an underlying condition, most often heart disease, obesity or metabolic disorders; among children, 41.3%, most often asthma or obesity.
• Pneumonia and influenza deaths had been at or above the epidemic threshold for 3 weeks.
• Children's deaths: 28 flu-associated pediatric deaths had been reported. Among the 22 eligible children with known vaccination status, only six had received any flu vaccine.

Percentage of outpatient visits for influenza-like illness by week, compared with recent seasons. CDC figure.

First in-season burden estimate: for the first time, CDC estimated the season's toll as it happened — from October 1 to February 2, 13,200,000–15,200,000 illnesses, 6,170,000–7,220,000 medical visits, 155,000–186,000 hospitalizations and 9,600–15,900 deaths.

What CDC recommended

• Vaccination for everyone 6 months and older as long as flu circulates. Interim estimates put vaccine effectiveness at 47% overall against medically attended flu and 46% against H1N1. In 2017–18, vaccination prevented an estimated 7.1 million illnesses, 3.7 million medical visits, 109,000 hospitalizations and 8,000 deaths.
• Antiviral treatment as soon as possible for people with confirmed or suspected flu who are severely ill, hospitalized or at high risk of complications, regardless of vaccination status — without relying on less sensitive rapid antigen tests to decide. Four antivirals were approved and recommended: oseltamivir, zanamivir, peramivir and baloxavir (approved October 24, 2018).

Sources

• Blanton L, Dugan VG, Abd Elal AI, et al. "Update: Influenza Activity — United States, September 30, 2018–February 2, 2019." Morbidity and Mortality Weekly Report 68(6), Centers for Disease Control and Prevention.