2018 Influenza Activity

On October 26, 2018, the CDC published a report that found between May 20, 2018, and October 13, 2018, low levels of influenza activity were reported in the United States, with a mix of influenza A and B viruses circulating. The CDC recommended that all persons six months old or older who do not have contraindications receive influenza vaccine.


In addition, a small number of nonhuman influenza variant virus infections were reported in the United States. Most were associated with exposure to swine. Although limited human-to-human transmission might have occurred in one instance, no ongoing community transmission was identified. The CDC recommended that vulnerable populations, especially young children and other persons at high risk for serious influenza complications, should avoid swine barns at agricultural fairs, or close contact with swine.


Between May 20, 2018, and October 13, 2018, U.S. clinical laboratories tested 197,295 respiratory specimens for influenza. Of those specimens, 2,763 (1.4 percent) were positive, including 1,801 (65.2 percent) that were positive for influenza A viruses and 962 (34.8 percent) that were positive for influenza B viruses. Public health laboratories in the United States tested 5,863 respiratory specimens for influenza viruses. Among these, 587 were positive for seasonal influenza viruses, including 442 (75.34 percent) positive for influenza A viruses and 145 (24.7 percent) for influenza B viruses. Influenza B viruses were more commonly detected than influenza A viruses from May until mid-June, whereas influenza A predominated from late June onward. A total of 400 (90.5 percent) of the seasonal influenza A viral specimens were subtyped by public health laboratories. Among these, 233 (58.3 percent) were influenza A(H1N1)pdm09, and 167 (41.8 percent) were influenza A(H3N2). Of the 118 (81.4 percent) influenza B viruses for which lineage was determined, 94 (79.7 percent) belonged to the B/Yamagata lineage and 24 (20.3 percent) to the B/Victoria lineage.


The CDC received reports of a small number of influenza outbreaks during the summer, including domestic origin outbreaks along with influenza virus infection identified in returning international travelers. Between May 20, 2018, and October 13, 2018, data indicated that the weekly percentage of outpatient visits to health care providers for influenza-like illness (ILI) remained below the national baseline of 2.2 percent, ranging from 0.6 to 1.4 percent. All regions remained below their region-specific ILI baselines. During the first two weeks of October, ILI activity levels for all reporting jurisdictions were minimal and, although a small number of jurisdictions have reported the geographic spread of influenza activity as local, approximately 60 percent of all reporting jurisdictions reported sporadic activity. Data indicated that the percentage of deaths attributed to pneumonia and influenza remained below the epidemic threshold during this period. Of the 183 influenza-associated pediatric deaths reported to the CDC that occurred during the 2017–18 influenza season, five occurred between May 20, 2018, and September 29, 2018. The first influenza-associated pediatric death occurring during the 2018–19 season was reported to the CDC in mid-October.


Fourteen human infections with novel influenza A viruses were reported in the United States between May 20, 2018, and October 13, 2018. Influenza viruses that normally circulate in swine and not humans are called variant viruses when detected in humans and designated with the letter v after the subtype. One infection was associated with an influenza A(H3N2)v virus, and 13 were associated with influenza A(H1N2)v viruses. All but one infection occurred among persons under the age of 18. The A(H3N2)v virus infection was reported from Indiana in a patient who reported swine contact at an agricultural fair in the week before symptom onset. All A(H1N2)v virus infections were reported in August from three states: California (six cases), Ohio (four), and Michigan (three). Eleven of the 13 patients reported contact with swine at agricultural fairs, one reported attendance at an agricultural fair but no contact with swine, and one reported neither contact with swine nor attendance at an agricultural fair. Limited human-to-human transmission might have taken place with this last A(H1N2)v infection. However, no ongoing or sustained human-to-human transmission associated with any of these infections was identified. None of the novel influenza A virus infections resulted in hospitalization, and all patients recovered. Studies indicate that vaccination with the 2017–18 seasonal influenza vaccine might offer less protection against this A(H3N2)v virus for children than adults. Studies indicate that vaccines specially developed to prevent A(H1N2)v virus infections would be protective. However, vaccination with the seasonal vaccine would not offer any protection.


Annual influenza vaccination is the best method for preventing influenza illness and its complications. The CDC recommended that all persons six months old or older who do not have contraindications receive influenza vaccine, preferably before the onset of influenza circulation in their community, which often begins in October and peaks sometime between December and February. The CDC recommended that health care providers offer vaccination by the end of October and continue to recommend and administer influenza vaccine to previously unvaccinated patients throughout the 2018–19 influenza season.


The components for the Northern Hemisphere 2018–19 influenza vaccines were selected in February 2018. The recommended Northern Hemisphere 2018–19 trivalent influenza vaccine composition included an A/Michigan/45/2015 (H1N1)pdm09-like virus, an A/Singapore/INFIMH-16–0019/2016 (H3N2)-like virus, and a B/Colorado/06/2017-like virus (B/Victoria lineage), with an additional influenza B virus (B/Phuket/3073/2013-like [B/Yamagata lineage]) recommended for quadrivalent vaccines.


The WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at the CDC tested 347 influenza virus specimens collected between May 20, 2018, and October 13, 2018, from the United States and worldwide for resistance to oseltamivir, peramivir, and zanamivir, the influenza virus neuraminidase inhibitor antiviral medications currently approved for use against seasonal influenza. Among 134 influenza A(H1N1)pdm09 viruses, 132 influenza A(H3N2) viruses, and 81 influenza B viruses tested, all were susceptible to all three medications. High levels of resistance to the adamantanes (amantadine and rimantadine) persisted among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses, which is consistent with the current recommendation to avoid use of these medications against influenza at this time.


Influenza antiviral medications can serve as a valuable adjunct to annual influenza vaccination. Early treatment with influenza antiviral medication is recommended for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at high risk for influenza-related complications. Early treatment has been shown to decrease time to symptom improvement and to reduce secondary complications associated with influenza. Providers should not delay treatment until test results become available because treatment is most effective when given early in the illness, especially within 48 hours of symptom onset. Providers should also not rely on less sensitive assays such as rapid antigen detection influenza diagnostic tests to inform treatment decisions.


See the CDC Report


See the CDC’s weekly influenza surveillance reports for the United States


See the CDC’s additional information regarding influenza viruses, influenza surveillance, influenza vaccines, influenza antiviral medications, and novel influenza A virus infections in humans


See also Medical Risk Law Report: Vaccines: An Ounce of Prevention May Lead to a Pound of Injury


See also Medical Risk Law Report: Drugs, Dosage, and Damage: Physician Liability for Prescribing or Administering Medication