The 2018–19 influenza season in the United States (September 30, 2018–May 18, 2019) was of moderate severity. Influenza-like illness (ILI) began rising in November, peaked in mid-February and fell below baseline in mid-April. At 21 weeks, it was the longest season in 10 years.
What set it apart was two waves of influenza A of similar size:
| Period | Main virus |
|---|---|
| October 2018 – mid-February 2019 | A(H1N1)pdm09 |
| Mid/late February – mid-May 2019 | A(H3N2) |
Influenza B was scarce — just 4% of viruses reported by public health labs. Compared with 2017–18, hospitalization rates were lower for adults but similar for children.
Lab testing
- Clinical labs tested 1,145,555 specimens; 177,039 (15.5%) were positive — 167,529 for influenza A and 9,510 for B. Weekly positivity ranged from 1.7% to 26.2% and peaked (25.1%–26.2%) in the weeks ending February 9–March 16.
- Public health labs tested 80,993 specimens; 42,303 (52.2%) were positive — 96.0% influenza A. Of B viruses with lineage data, 63.3% were B/Victoria and 36.7% B/Yamagata.
H1N1pdm09 was the most common virus among people aged 0–4 (57.1%) and 25–64 (63.2%), and H3N2 among those 5–24 (48.8%) and 65 and older (51.3%).
How well the viruses matched the vaccine
CDC genetically characterized 2,750 viruses and tested a subset for how well antibodies against the 2018–19 vaccine viruses recognized them.
| Virus | Finding |
|---|---|
| A(H1N1)pdm09 | All belonged to subclade 6B.1A; 318 of 331 (96.1%) were well recognized by antibodies to the vaccine reference virus |
| A(H3N2) | Clade 3C.3a rose from 12.7% of viruses by November 2018 to 81.9% from December to May. Only 191 of 505 (37.8%) were well recognized by antibodies to the cell-grown vaccine reference virus, and 43 of 388 (11%) by those to the egg-grown version, probably because of egg-adaptive changes. Of the 314 poorly recognized viruses, 312 (99.4%) were clade 3C.3a |
| B/Yamagata | All 178 tested were well recognized |
| B/Victoria | Several distinct groups circulated; 147 of 191 were well recognized. Poorly recognized ones included the V1A-3Del subclade, more common late in the season |
Antiviral resistance
Of 2,699 viruses tested against oseltamivir, zanamivir and peramivir:
- 5 H1N1pdm09 viruses carried the H275Y change and were far less inhibited by oseltamivir and peramivir; 4 more showed some reduced inhibition by oseltamivir;
- 2 influenza B viruses carried H273Y and were far less inhibited by peramivir.
This season CDC also began testing for resistance to baloxavir: none of 2,673 viruses had resistance-linked changes, and all 191 tested in the lab were susceptible. Overall, more than 99% of viruses were susceptible to oseltamivir and peramivir, and all to zanamivir and baloxavir.
The 2019–20 vaccine
WHO and the FDA's vaccine advisory committee recommended:
| Component | 2019–20 virus |
|---|---|
| A(H1N1)pdm09 | A/Brisbane/02/2018-like — updated |
| A(H3N2) | A/Kansas/14/2017-like (clade 3C.3a) — updated |
| B/Victoria | B/Colorado/06/2017-like |
| B/Yamagata (four-strain vaccines only) | B/Phuket/3073/2013-like |
The H1N1 part changed because blood from vaccinated people showed eightfold or greater lower antibody levels against recent 6B.1A viruses than against the old vaccine virus; the H3N2 part changed to catch up with antigenic drift as 3C.3a viruses spread.
Illness, hospitalization and death
- Outpatient visits for ILI stayed at or above the 2.2% national baseline for 21 straight weeks. In the week ending February 23, 33 of 54 jurisdictions reported high ILI activity and 50 reported widespread flu.
- Hospitalizations: the FluSurv-NET system, covering about 27 million people, recorded 18,847 lab-confirmed flu hospitalizations from October 1 to April 30 — 65.3 per 100,000. Adults 65 and older made up about 47%.
| Age | Hospitalizations per 100,000 |
|---|---|
| 0–4 | 72.0 |
| 5–17 | 20.4 |
| 18–49 | 25.8 |
| 50–64 | 80.7 |
| 65+ | 221.7 |
- Among hospitalized adults with records, 92.6% had at least one high-risk condition — most often heart disease (45.0%), metabolic disorders (42.9%), obesity (39.4%) and chronic lung disease (29.9%). Among children, 55.0% did, most often asthma (27.1%) and neurologic disorders (14.7%).
- Deaths from pneumonia and influenza were at or above the epidemic threshold for 10 weeks, peaking twice at 7.7%.
- 116 children died of lab-confirmed flu (mean age 6.1). Of 104 with known histories, 53 (51%) had a high-risk condition. Of 89 eligible children with known vaccination status, only 30 (34%) had received any flu vaccine.
Compared with other seasons
| Season | Hospitalizations per 100,000 |
|---|---|
| 2014–15 | 64.1 |
| 2016–17 | 62.0 |
| 2017–18 | 102.9 |
| 2018–19 | 65.3 |
CDC's preliminary estimate for 2018–19 was 37.4–42.9 million illnesses, 17.3–20.1 million medical visits, 531,000–647,000 hospitalizations and 36,400–61,200 deaths — within the range seen each season since 2010–11 (9.3–49 million illnesses and 12,000–79,000 deaths).
Advice
- A yearly flu vaccine is the best protection.
- Flu is often reported in summer too: suspect it in sick travelers returning from countries with flu activity, and refer suspected variant flu from swine at animal exhibitions to state health departments.
- Start antivirals as soon as possible for people with confirmed or suspected flu who are severely ill, hospitalized or at high risk. Four are FDA-approved for uncomplicated flu within 2 days of onset: oseltamivir, zanamivir, peramivir and baloxavir. Don't rely on less sensitive rapid antigen tests for treatment decisions.
Sources
Based on Xu X, Blanton L, Abd Elal AI, et al., "Update: Influenza Activity in the United States During the 2018–19 Season and Composition of the 2019–20 Influenza Vaccine," Morbidity and Mortality Weekly Report 68(24), CDC; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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