Influenza, Summer 2018: Low U.S. Activity, a Mild Southern Season, and Swine Flu at Fairs

Summary

• From May 20 to October 13, 2018, influenza activity in the United States was low, as usual for the time of year, with a mix of influenza A and B.
• The Southern Hemisphere season was relatively low and mild, mostly A(H1N1)pdm09.
• Testing found no significant antigenic drift in circulating viruses.
• Fourteen people were infected with "variant" viruses — flu viruses that normally circulate in pigs — mostly after contact with swine at fairs.
• Everyone 6 months and older without a contraindication should get a flu vaccine — ideally by the end of October, before flu spreads.

In the United States

The U.S. flu surveillance system — CDC working with federal, state, local and territorial partners — draws on eight data sources, six of them year-round.

• Clinical labs tested 197,295 specimens; 2,763 (1.4%) were positive — 65.2% influenza A, 34.8% influenza B.
• Public health labs tested 5,863; 587 were positive for seasonal flu, mostly A. B led from May to mid-June, A from late June on. Of subtyped A viruses, 58.3% were A(H1N1)pdm09 and 41.8% H3N2; of B viruses with a known lineage, 79.7% were Yamagata and 20.3% Victoria.
• Doctor visits for flu-like illness stayed below the national baseline of 2.2% (0.6%–1.4%), and below baseline in every region. In early October, activity was minimal everywhere, with about 60% of jurisdictions reporting sporadic flu.
• Deaths from pneumonia and influenza stayed below the epidemic threshold. Of 183 flu deaths in children in the 2017–18 season, five occurred from May 20 to September 29; the first child death of 2018–19 was reported in mid-October.
• A few summer outbreaks occurred, some in returning travelers.

Figure 1. Positive influenza tests at U.S. clinical laboratories, by type and week. Figure from the CDC report.

Figure 2. Positive influenza tests at U.S. public health laboratories, by type, subtype or lineage, and week. Figure from the CDC report.

Around the world

CDC is one of six WHO Collaborating Centers in the Global Influenza Surveillance and Response System.

• Temperate South America: rose from mid-May, peaked in August; H3N2 dominated in Chile and Paraguay.
• Temperate southern Africa: rose in April, peaked in June with A(H1N1)pdm09; a second wave, mostly B, peaked in September.
• Australia and New Zealand: below seasonal threshold, mostly A(H1N1)pdm09.
• Tropical regions: more variable, A(H1N1)pdm09 mostly dominant; all three types in Eastern Africa; H1N1 and H3N2 in Southern Asia.

The viruses

CDC characterized 680 viruses from the U.S. and abroad (351 A(H1N1)pdm09, 185 H3N2, 144 B), compared with the strains in the 2018–19 Northern Hemisphere vaccine — A/Michigan/45/2015 (H1N1)pdm09, A/Singapore/INFIMH-16-0019/2016 (H3N2), B/Colorado/06/2017 (Victoria), plus B/Phuket/3073/2013 (Yamagata) in four-strain vaccines.

• A(H1N1)pdm09: all in group 6B.1, and all tested were antigenically similar to the vaccine strain.
• H3N2: several groups circulated (3C.2a, 3C.2a1, 3C.3a), mostly 3C.2a, largely subclade 3C.2a2. 91.9% reacted well with antisera to the cell-grown vaccine strain — but antisera to the egg-grown version inhibited fewer, while antisera to egg-grown A/Switzerland/8060/2017 inhibited most 3C.2a2 viruses.
• B/Victoria: 86.1% in subgroup V1A.1 (with a two–amino acid deletion), like the vaccine strain; one had a three–amino acid deletion, a type seen sporadically abroad. 18 of 19 reacted well with the vaccine strain's antisera.
• B/Yamagata: all in group Y3, and all reacted well.

Figure 3. Genetic characterization of viruses collected May 20–October 13, 2018. Figure from the CDC report.

Antivirals: all 347 viruses tested were susceptible to oseltamivir, peramivir and zanamivir. Resistance to the adamantanes (amantadine and rimantadine) remained high, so they should not be used.

The 2019 Southern Hemisphere vaccine

At a WHO meeting in Atlanta on September 24–27, 2018, the recommended strains were A/Michigan/45/2015 (H1N1)pdm09, A/Switzerland/8060/2017 (H3N2) and B/Colorado/06/2017 (Victoria), plus B/Phuket/3073/2013 (Yamagata) for four-strain vaccines. Compared with the Northern Hemisphere 2018–19 vaccine, the only change is the H3N2 strain for egg-based vaccines — made not because the virus had drifted, but to offset changes H3N2 undergoes when grown in eggs. Cell-grown and recombinant vaccines keep A/Singapore.

Swine flu variants

Fourteen people caught novel influenza A viruses of swine origin: one A(H3N2)v (Indiana) and 13 A(H1N2)v in August — California (6), Ohio (4), Michigan (3). All but one were under 18. Twelve had touched pigs at agricultural fairs; one had been to a fair without touching pigs, and one had done neither — where limited person-to-person spread may have occurred, though no sustained spread was found. No one was hospitalized, and all recovered.

• The A(H3N2)v virus resembled 2017 variants and U.S. swine viruses. Adults vaccinated in 2017–18 had antibodies that reacted reasonably well to it, but children's reacted less — so the seasonal vaccine may protect children less than adults.
• The A(H1N2)v viruses matched pigs at the fairs. A candidate vaccine virus made for them would protect, but the seasonal vaccine would offer none.

Young children and others at high risk should avoid swine barns at fairs and close contact with pigs. Clinicians should consider novel flu in people with flu-like illness after contact with pigs or poultry, or severe respiratory illness after travel to places with bird flu.

Get vaccinated, treat early

The 2017–18 season caused an estimated 79,000 deaths and 960,000 hospitalizations. Even at 40%–60% effectiveness, flu vaccination prevents millions of medical visits, tens of thousands of hospitalizations and thousands of deaths each year. A 2017 CDC study found vaccination cut the risk of flu death by 51% in children with high-risk conditions and 65% in healthy children, and an August 2018 study found it reduced severe illness, hospitalization and intensive care in adults. That matters most for people 65 and older, children under 5, pregnant women, and people with conditions such as heart or lung disease or diabetes.

• Timing: by the end of October, but keep offering it all season.
• Children 6 months–8 years need 2 doses, at least 4 weeks apart, unless they had at least 2 doses before July 1, 2018.
• No vaccine is preferred over another; manufacturers projected 163 to 168 million U.S. doses.
• Antivirals: treat early — especially within 48 hours — anyone with suspected or confirmed flu who is severely ill, hospitalized or at high risk. Don't wait for test results, and don't rely on less sensitive rapid antigen tests.

Weekly updates: cdc.gov/flu/weekly.

Sources

Based on Eric J. Chow, C. Todd Davis, Anwar Isa Abd Elal and others, "Update: Influenza Activity — United States and Worldwide, May 20–October 13, 2018," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.