Skip to main content

Military Health System

Important Notice about Pharmacy Operations

Change Healthcare Cyberattack Impact on MHS Pharmacy Operations. Read the statement to learn more. 

Summary of the 2018–2019 Influenza Season Among Department of Defense Service Members and Other Beneficiaries

Image of 02_flu shot_DVIDS. A flu shot vaccination sits on a table at 184th Sustainment Command headquarters in Monticello, Mississippi on Feb. 8, 2020. The single best way to prevent seasonal flu is to get vaccinated each year, but good wellness habits like covering your cough and washing your hands often can help prevent the spread of germs. (Mississippi Army National Guard photo by Staff Sgt. Veronica McNabb)

What Are the New Findings?

The 2018–2019 influenza season was longer than the preceding 2 seasons. Unlike most prior seasons, 2 strains were common. Influenza A(H1N1)pdm09 was the most common strain early in the season, but influenza A(H3N2) predominated later in the season. Total influenza vaccine effectiveness was low during this season in part because the A(H3N2) strain was antigenically drifted from the vaccine strain.

What Is the Impact on Readiness and Force Health Protection?

Surveillance data about influenza disease inform the planning and strategy for efforts to reduce the future impact of influenza on the health and medical readiness of the Armed Forces. The data and findings in this report reinforce the importance of the use of up-to-date multivalent influenza vaccines that protect against several different specific virus strains that may become common in the coming influenza season.


The Armed Forces Health Surveillance Branch conducts weekly surveillance of influenza activity among Department of Defense (DOD) populations each influenza season. This report provides a summary of the data from the 2018–2019 influenza season. Ambulatory data for influenza-like illnesses (ILIs), influenza hospitalization data, and lab data for influenza-confirmed cases were used for the surveillance. The 2018–2019 season differed from past seasons in that it was much longer, had a later peak, and the predominant strain of influenza changed from influenza A(H1N1)pdm09 at the beginning of the season to influenza A(H3N2) in the middle of the season. Non-service member beneficiaries accounted for the majority of ILI-related encounters and hospitalizations. However, there were still 149 influenza-related hospitalizations among service members during the 2018–2019 season. Continued weekly surveillance of influenza among DOD populations is crucial to track increases in activity each season and the potential emergence of new and/or severe influenza subtypes.


Influenza infects an estimated 8% of the U.S. population annually, with children and the elderly at highest risk.1 Service members may also be at a higher risk for exposure to influenza because of increased crowding and mixing in the recruit setting and duty assignments abroad where influenza subtypes may differ.2 Each influenza season is different because of antigenic drift in the circulating influenza subtypes, the degree of match between vaccine subtypes and circulating subtypes, and vaccine coverage of the population. As such, it is important to conduct annual surveillance of each influenza season to identify the onset and patterns of activity, emergence of drifted or shifted subtypes, and severity of the season.

The Armed Forces Health Surveillance Branch of the Defense Health Agency utilizes electronic sources of ambulatory medical encounters, hospitalizations, and laboratory data to conduct annual influenza surveillance among all Department of Defense (DOD) beneficiaries across the world. Weekly reports are generated to provide near real-time influenza surveillance data for each of the DOD Combatant Commands. This report provides a summary of DOD influenza surveillance data for the 2018–2019 influenza season.


Medical encounter and demographic data from the Defense Medical Surveillance System (DMSS) and Health Level 7 (HL7)-formatted laboratory data from the Navy and Marine Corps Public Health Center (NMCPHC) were used for this analysis. The HL7-formatted laboratory data are non standardized, so NMCPHC applies an algorithm to the data to identify influenza tests and standardize results. The surveillance period for the 2018–2019 influenza season was 30 Sept. 2018 through 1 June 2019 (influenza weeks 40 through 22). Data from the 2016–2017 and 2017–2018 influenza seasons are also presented for comparison. The surveillance population included all individuals who were Military Health System (MHS) beneficiaries (i.e., active and reserve/guard component service members, retired service members, family members and other dependents of service members and retirees, and other authorized government employees and family members) who accessed care through either a military medical facility/provider or a civilian facility/provider (if paid for by the MHS). However, medical data from military treatment facilities (MTFs) that were using MHS GENESIS at the time of this surveillance (Naval Hospital Oak Harbor, Naval Hospital Bremerton, Air Force Medical Services Fairchild, and Madigan Army Medical Center) are not captured in the DMSS data. Therefore, medical encounter and laboratory data from these MTFs are not included in the analysis. For the analysis, populations were grouped as service members or other beneficiaries.

Outpatient medical encounters were classified as an influenza-like illness (ILI) encounter if they had an ILI diagnosis code (International Classification of Diseases, 10th Revision [ICD-10] codes B97.89, H66.9, H66.90, H66.91, H66.92, H66.93, J00, J01.9, J01.90, J06.9, J09, J09.X, J09.X1, J09.X2, J09.X3, J09.X9, J10, J10.0, J10.00, J10.01, J10.08, J10.1, J10.2, J10.8, J10.81, J10.82, J10.83, J10.89, J11, J11.0, J11.00, J11.08, J11.1, J11.2, J11.8, J11.81, J11.82, J11.83, J11.89, J12.89, J12.9, J18, J18.1, J18.8, J18.9, J20.9, J40, R05, R50.9) in any diagnostic position. The percentage of all outpatient encounters that were classified as ILI encounters was calculated for each week for each study population. Baseline ILI activity for the season was defined as the mean percentage of all outpatient encounters during non influenza weeks (weeks 22–39) over the prior 3 years.

Hospitalized influenza cases were defined as having a hospitalization with a diagnosis of influenza (ICD-10: J09, J10, J11) in any diagnostic position. The number of hospitalized influenza cases each week for each study population was calculated. For other beneficiaries, counts of influenza hospitalizations by age group (0–4, 5–9, 10–17, 18–35, 36–49, 50–64, 65+) were calculated.

Laboratory-confirmed influenza cases were defined as having a positive polymerase chain reaction, viral culture, or rapid influenza assay result. Laboratory-confirmed influenza cases were stratified by influenza types/subtypes (influenza A (not subtyped), influenza A(H1N1)pdm09, influenza A(H3N2), influenza A and B coinfection, and influenza B. The total number of laboratory-confirmed influenza cases stratified by type/subtype and the percentage of all influenza laboratory tests performed that had positive test results were calculated for each week of the influenza season for service members and for other beneficiaries separately.


Virus surveillance

Among all beneficiaries, there were 149,254 respiratory specimens tested for influenza during the 2018–2019 influenza season (data not shown). Of those, 30,464 (20.4%) were positive for influenza. Service members had a lower percentage of specimens testing positive for influenza (16.7%) compared to other beneficiaries (21.8%). Among all populations, influenza A (any subtype) predominated during this season, with 28,454 (93.4%) of all positive specimens testing positive for influenza A. The distribution of subtypes among influenza A positive specimens was 73.3% influenza A (not subtyped), 12.6% A(H3N2), and 7.5% A(H1N1)pdm09. The remaining specimens were positive for influenza B (1,805; 5.9%) or an influenza A/B coinfection (205; 0.7%). The distribution of subtypes was similar between service members and other beneficiaries (data not shown).

The distribution of influenza serotypes and the percentage of specimens positive for influenza by week are presented in Figures 1a and 1b for service members and other beneficiaries, respectively. Among subtyped influenza A specimens, A(H1N1) pdm09 predominated early in the season, but A(H3N2) was predominant after week 3. The highest numbers of positive specimens and the highest percentages of positives occurred during week 9 for service members and weeks 6 and 7 for other beneficiaries. These results indicate peak influenza activity for the season during the month of Feb. 2019.

Outpatient encounter ILI surveillance

During the 2018–2019 season, the weekly percentages of outpatient encounters due to an ILI for service members were above baseline (2.1%) for 22 weeks (weeks 46–15) (Figure 2a). A similar pattern was seen among other beneficiaries, for whom the percentages were above baseline (3.4%) for 20 weeks (weeks 47–14) (Figure 2b). This pattern is similar to the percentage of outpatient encounters due to ILI during the prior 2 influenza seasons.

Earlier in the 2018–2019 season, between weeks 40–52, the trend and magnitude of the percentages of encounters due to ILI were also similar to those of the past 2 seasons (Figures 2a and 2b). All seasons had peaks during weeks 52 and 1. This timing coincides with the end-of-year holiday period. Rather than a true peak in ILI activity though, this peak was being driven by a differential decrease in the total number of medical encounters and ILI encounters during that time. Specifically, for the 2018–2019 season, the total number of outpatient medical encounters decreased 58% from week 51 to week 52; however, ILI encounters decreased only 36% between those 2 weeks. Therefore, this peak in ILI percentage is considered an artifact of the overall decline in total outpatient encounters and is not reflected in the peak influenza weeks for the season. After week 1, the 2018–2019 season ILI percentages began to diverge from the prior 2 seasons. Among service members, the percentage of encounters due to ILI had a later peak (week 8) than the prior 2 seasons (weeks 2 and 3), but the magnitude of the 2018–2019 peak was similar to that of the 2017–2018 peak (Figure 2a). Among other beneficiaries, the trend was similar to the 2 prior seasons, with peak activity occurring during week 6 (2017–2018: week 5; 2016–2017: week 6), and the magnitude was similar to the 2016–2017 season (Figure 2b).

Influenza-related hospitalizations

Of the total 5,847 influenza-related hospitalizations during the 2018–2019 season, 149 occurred among service members (Figure 3). The majority of hospitalizations occurred among other beneficiaries (n=5,698; 97.5%). Hospitalizations peaked overall during week 11 (n=471), but service member hospitalizations peaked during week 10 (n=18) (Figure 3). Among other beneficiaries, the majority of influenza-related hospitalizations occurred among those 65 years of age or older (n=3,778; 66.3%) (Figure 4).

Editorial Comment

The 2018–2019 influenza season among service members and other DOD beneficiaries was a longer season with a later peak compared to the prior 2 seasons. The season also differed from prior seasons in that the beginning of the season was predominated by influenza A(H1N1)pdm09 while influenza A(H3N2) predominated after week 3; most seasons have just 1 influenza A subtype predominating. As expected, the influenza season among DOD service members and beneficiaries was similar to the season among the general U.S. population.3 Although the DOD influenza surveillance data include information from around the world, the majority of encounter and laboratory data came from the U.S. and to a lesser extent Europe, which also had an influenza season similar to that in the U.S.4 As with the general U.S. population, the elderly (> 64 years of age) accounted for the majority of influenza hospitalizations among other beneficiaries. The elderly population accounted for 66% of all other beneficiary hospitalizations for the season compared to 47% among the general U.S. population.3

A seasonal influenza vaccine is still the best way to protect against influenza. Service members are required to receive a seasonal influenza vaccine annually. During the 2018–2019 season, DOD policy set a goal of 90% of service members vaccinated by 15 Jan. 2019.5 Although vaccination rates of service members were very high, influenza cases still occurred among this population during the 2018–2019 season. Cases of influenza among service members may be attributable to infections occurring before receipt of the influenza vaccine, within the 14 days following vaccination when the vaccine may not provide complete protection, or after vaccination because the vaccine is less than 100% effective. During the 2018–2019 season, vaccine effectiveness among the general U.S. population was particularly low because of the emergence of a drifted A/H3N2 (clade 3C.3a) circulating virus that differed from the vaccine strain.6 Although the influenza vaccine is not 100% effective at preventing influenza infection, a recent study showed that vaccination also decreased the risk of hospitalization and admission to the intensive care unit and decreased severity of illness.7 Continued vaccination of service members and other DOD beneficiaries is crucial to combat influenza infections and lessen disease severity. This season also demonstrated the importance of annual influenza surveillance, as the seasons differ from year to year.


  1. Tokars JI, Olsen SJ, Reed C. Seasonal incidence of symptomatic influenza in the United States. Clin Infect Dis. 2018;66(10):1511–1518.
  2. Sanchez JL, Cooper MJ. Influenza in the US military: an overview. J Infec Dis Treat. 2016;2(1).
  3. Xu X, Blanton L, Elal AIA, et al. Update: Influenza activity in the United States during the 2018–19 season and composition of the 2019–20 influenza vaccine. MMWR Morb Mortal Wkly Rep. 2019;68(24):544–551.
  4. European Centre for Disease Prevention and Control. Weekly influenza update, week 20, May 2019. Accessed 28 Jan. 2020.
  5. Department of Defense Assistant Secretary of Defense. Memorandum: Guidance for the 2018–2019 Annual Influenza Immunization Program. 05 July 2018.
  6. Flannery B, Kondor RJG, Chung JR, et al. Spread of antigenically drifted influenza A(H3N2) viruses and vaccine effectiveness in the United States during the 2018–2019 season. J Infect Dis. 2020;221(1):8–15.
  7. Thompson MG, Pierse N, Sue Huang Q, et al. Influenza vaccine effectiveness in preventing influenza-associated intensive care admissions and attenuating severe disease among adults in New Zealand 2012–2015. Vaccine. 2018;36(39):5916–5925.


FIGURE 1a. Numbers of laboratory-confirmed influenza specimens by serotype and percentages of respiratory specimens positive for influenza by surveillance week, service members, U.S. Armed Forces, 2018–2019 influenza season

FIGURE 1b. Numbers of laboratory-confirmed influenza specimens by serotype and percentages of respiratory specimens positive

FIGURE 2a. Percentages of outpatient encounters due to ILI, service members, U.S. Armed Forces, 2018–2019 influenza season

FIGURE 2b. Percentages of outpatient encounters due to ILI, other DoD beneficiaries, 2018–2019 influenza season

IGURE 3. Influenza-related hospitalizations, service members and other DoD beneficiaries, 2018–2019 influenza season

FIGURE 4. Age distribution of beneficiaries with influenza-related hospitalizations, 2018–2019 influenza season

You also may be interested in...

Jun 1, 2022

Medical Evacuations out of the U.S. Central and U.S. Africa Commands, Active and Reserve Components, U.S. Armed Forces, 2021

The proportions of evacuations out of USCENTCOM that were due to battle injuries declined substantially in 2021. For USCENTCOM, evacuations for mental health disorders were the most common, followed by non-battle injury and poisoning, and signs, symptoms, and ill-defined conditions. For USAFRICOM, evacuations for non-battle injury and poisoning were ...

Jun 1, 2022

Ambulatory Visits, Active Component, U.S. Armed Forces, 2021

In 2021, the overall numbers and rates of active component service member ambulatory care visits were the highest of any of the last 10 years. Most categories of illness and injury showed modest increases in numbers and rates. The proportions of ambulatory care visits that were accomplished via telehealth encounters fell to under 15% in 2021, compared ...

Jun 1, 2022

Absolute and Relative Morbidity Burdens Attributable to Various Illnesses and Injuries, Active Component, U.S. Armed Forces, 2021

In 2021, as in prior years, the medical conditions associated with the most medical encounters, the largest number of affected service members, and the greatest number of hospital days were in the major categories of injuries, musculoskeletal disorders, and mental health disorders. Despite the pandemic, COVID-19 accounted for less than 2% of total ...

May 1, 2022

The Association Between Two Bogus Items, Demographics, and Military Characteristics in a 2019 Cross-sectional Survey of U.S. Army Soldiers

NIANTIC, CT, UNITED STATES 06.16.2022 U.S. Army Staff Sgt. John Young, an information technology specialist assigned to Joint Forces Headquarters, Connecticut Army National Guard, works on a computer at Camp Nett, Niantic, Connecticut, June 16, 2022. Young provided threat intelligence to cyber analysts that were part of his "Blue Team" during Cyber Yankee, a cyber training exercise meant to simulate a real world environment to train mission essential tasks for cyber professionals. (U.S. Army photo by Sgt. Matthew Lucibello)

Data from surveys may be used to make public health decisions at both the installation and the Department of the Army level. This study demonstrates that a vast majority of soldiers were likely sufficiently engaged and answered both bogus items correctly. Future surveys should continue to investigate careless responding to ensure data quality in ...

May 1, 2022

Update: Sexually Transmitted Infections, Active Component, U.S. Armed Forces, 2013–2021

This illustration depicts a 3D computer-generated image of a number of drug-resistant Neisseria gonorrhoeae bacteria. CDC/James Archer

This report summarizes incidence rates of the 5 most common sexually transmitted infections (STIs) among active component service members of the U.S. Armed Forces during 2013–2021. In general, compared to their respective counterparts, younger service members, non-Hispanic Black service members, those who were single and other/unknown marital status, ...

Mar 1, 2022

Brief report: Using syndromic surveillance to monitor MIS-C associated with COVID-19 in Military Health System beneficiaries

Air Force 1st Lt. Anthony Albina, a critical care nurse assigned to Joint Base Andrews, Md., checks a patient’s breathing and heart rate during an intubation procedure while supporting COVID-19 response operations in Cleveland, Jan. 20, 2022.

SARS CoV-2 and the illness it causes, COVID-19, have exacted a heavy toll on the global community. Most of the identified disease has been in the elderly and adults. The goal of this analysis was to ascertain if user-built ESSENCE queries applied to records of outpatient MHS health care encounters are capable of detecting MIS-C cases that have not ...

Mar 1, 2022

Surveillance Snapshot: Medical Separation from Service Among Incident Cases of Osteoarthritis and Spondylosis, Active Component, U.S. Armed Forces, 2016–2020

Marines hike to the next training location during Exercise Baccarat in Aveyron, Occitanie, France, Oct.16, 2021. Exercise Baccarat is a three-week joint exercise with Marines and the French Foreign Legion that challenges forces with physical and tactical training. Photo By: Marine Corps Lance Cpl. Jennifer Reyes

Osteoarthritis (OA) is the most common adult joint disease and predominantly involves the weight-bearing joints. This condition, including spondylosis (OA of the spine), results in significant disability and resource utilization and is a leading cause of medical separation from military service.

Last Updated: July 11, 2023
Follow us on Instagram Follow us on LinkedIn Follow us on Facebook Follow us on X Follow us on YouTube Sign up on GovDelivery