INFLUENZA

(FLU)

Overview

Influenza or "flu" is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. Symptoms characteristically start with sudden onset of fever, and may include cough, sore throat, runny or stuffy nose, headache, body aches and nausea. While people 65 years and older, young children, and people with underlying health conditions are at higher risk of serious influenza complications than the active-duty population, severe cases can occur unpredictably among active-duty personnel as well. Also, because it can spread quickly, influenza outbreaks may be widespread and can adversely impact Navy and Marine Corps force readiness and mission execution.

Seasonal vaccination is the most effective control measure to reduce the risk of severe influenza and mission degradation, minimizing the risk to force. Department of War (DoW) policy mandates that all Active Duty and Reserve Component personnel activated for more than 30 days be immunized against influenza annually.

Influenza A (H3N2) subclade K: International reports from Australia, Japan and the UK have indicated a worse than usual 2025-2026 influenza season. Years in which influenza type A (H3N2) predominate are typically more severe than other years, as was seen in the 2002-2003 and 2017-2018 seasons. Because influenza A (H3N2) seasons are often worse than others, immunization is especially important to ensure a healthy force.

This year, there has also been a genetic drift to the predominantly circulating subclade K. Despite media hype of “superflu,” there is no indication of severity beyond what is typically seen in a bad influenza A (H3N2) season. Immunization, despite the influenza vaccine being a suboptimal match to Subclade K, is still expected to retain an important degree of effectiveness against severe illness.

Policy and Guidance

Web Resources

Reporting

All Influenza outbreaks and influenza-associated hospitalizations of individuals under 65 years old are DoW Reportable Medical Events and should be documented in the Disease Reporting System internet (DRSi). Details may be found in the Armed Forces Reportable Medical Events Guide. In summary, a confirmed influenza-associated hospitalized case is defined as a clinically compatible illness with ALL of the following:

• Younger than 65 years of age and
• Any positive influenza laboratory test (example: culture, DFA, IFA, rapid, PCR)

AND also includes

• Hospital admissions ≤ 14 days after a positive influenza test or
• Hospital admissions ≤ 3 days before a positive influenza test

The patient’s influenza immunization history should be included as a critical reporting element, as is the virus type (if known).
For information on DRSi and Reporting, see the NMCFHPC ‘s Medical Surveillance and Reporting page. Hospitalized cases and outbreaks should be reported in Disease Reporting System internet (DRSi). Operational medical departments without ready access to the internet can report to their nearest MTF or NEPMU via phone, email, or message.
Use the Data Elements for Disease Reporting link to guide report generation. An Influenza Hospitalized Case Worksheet is available to facilitate data collection for hospitalized cases.

Public Health

Outbreaks of influenza are common in both military and civilian congregate settings. Ships and training centers are particularly vulnerable to rapidly spreading outbreaks that may result in training and operational disruptions. In the 1918 influenza outbreak, over 26,000 U.S. servicemembers died from influenza, with some estimates exceeding 40,000 deaths. DoW-required influenza immunization has been the cornerstone of influenza prevention and control since the 1950s, successfully minimizing severe morbidity and mortality among active-duty personnel for decades.
Immunization with Hemisphere (Northern or Southern) and Season-specific vaccine remains the most effective way to prevent influenza outbreaks and severe disease.

Surveillance

CDC routinely monitors influenza activity. As a result of regular surveillance, we know that in the U.S., peak influenza activity occurs between December and March, with February being the most common peak month. Trends this year suggest that 2025-2026 may be on the early side.

Routine surveillance reports:

AFHSD routine surveillance reports: .mil users can subscribe by emailing dha.ncr.health-surv.list.ib-alert-response@health.mil
CDC Weekly U.S. Influenza Surveillance Report
DHA Public Health Influenza Dashboard (*CAC Required*)

Ships and stations should monitor influenza trends in their supported populations and be prepared to implement control measures early in the event of an outbreak. Electronic Surveillance System for the Early Notification of Community-based Epidemics (ESSENCE) is a valuable and powerful tool to supplement surveillance activities and can be used to monitor and identify outbreaks on multiple platforms within an area of responsibility. A pre-built Respiratory Disease Dashboard in ESSENCE can be obtained from the nearest NEPMU. For more information on ESSENCE, visit NMCFHPC’s Medical Surveillance and Reporting page.

Various commands in the Defense Health Agency produce reports for specific populations that may inform local Navy and Marine Corps medical surveillance activities. These include:

DOW Global, Laboratory-based, Respiratory Surveillance Program: https://www.health.mil/Military-Health-Topics/Health-Readiness/AFHSD/Reports-and-Publications/Respiratory-Summary-and-Reports
Armed Forces Health Surveillance Division: https://www.health.mil/Military-Health-Topics/Health-Readiness/AFHSD/Integrated-Biosurveillance/Alert-and-Response-Operations

Active molecular surveillance through the WHO’s Global Influenza Surveillance and Response System looks for new strains of influenza. It is conducted in many countries throughout the world to guide the development of each year’s vaccine components. Within DoW, influenza strain distribution is monitored through Global Emerging Infections Surveillance (GEIS), which funds a network of global research laboratories. A description of DoW-GEIS respiratory surveillance activities was recently published in MSMR: Beyond the Clinic: The Importance of DoD Viral Respiratory Panel Testing for Public Health Surveillance. The primary influenza sequencing laboratory is the DoW Global Respiratory Pathogen Surveillance Program (*CAC Required*)

Managing an Outbreak

Despite near-complete immunization of active-duty forces, outbreaks of influenza are not uncommon in high-risk settings (e.g., ships, training environments). Widespread travel can introduce out-of-season outbreaks, and occasionally the circulating strain differs enough from the vaccine strain to decrease effectiveness.

Recommended actions to respond to an influenza outbreak include:

Initiate enhanced surveillance for febrile respiratory illness; a line list should be prepared and populated with confirmed and suspected cases.
Ensure appropriate clinical testing is being conducted; lab should be advised to send several specimens for full virus characterization.
- DoD Global Respiratory Pathogen Surveillance Program (*CAC Required)
- Influenza Specimen Collection
Initiate influenza vaccination if the population is unvaccinated (e.g., newly arrived accessions); acceleration of routine vaccinations may be indicated in training environments.
Institute non-pharmacologic interventions to reduce spread, including enhanced hygiene measures and isolation of ill personnel. Specific resources:
Report outbreaks through DRSi (see Reporting).
In high-risk training settings or ships, consider initiating Tamiflu (Oseltamivir) for chemoprophylaxis. Note that the chemoprophylaxis dose (75 mg once per day) is half the treatment dose (75 mg twice per day) and should be sustained for 7 days after the onset of the last case in the affected cohort.
Contact local Preventive Medicine (if available) for support. The cognizant Navy Environmental and Preventive Medicine Unit (NEPMU) can advise or assist with outbreak investigations in operational or training settings.

Epidemiology and Selected Publications

Aquino, T. L., Brice, G. T., Hayes, S., Myers, C. A., McDowell, J., White, B., ... & Johnston, D. (2014). Influenza outbreak in a vaccinated population—USS Ardent, February 2014. Morbidity and Mortality Weekly Report, 63(42), 947.

Mazumder, Abir; Ray, Sougat1; Bhaskar, Vijay2; Anand, Kavita B3,; Kumar, B Vijay4. Postexposure Prophylaxis with Oseltamivir in Outbreak Control of pH1N1 Influenza Onboard a Naval Warship: An Observational Study. Journal of Marine Medical Society 22(2):p 123-127, Jul–Dec 2020. | DOI: 10.4103/jmms.jmms_19_20

Pollett S, Hone E, Richard SA, Schmidt K, Simons MP, Wayman M, Tant R, Rothenberg J, Hogan V, O’Connell R, Burgess T, Fries AC, Tilley D, Colombo RE. P-724. The epidemiology, phenotype, and phylogeny of an influenza A/H3N2 virus outbreak among vaccinated U.S. Navy midshipmen. Open Forum Infect Dis. 2025 Jan 29;12(Suppl 1):ofae631.920. doi: 10.1093/ofid/ofae631.920. PMCID: PMC11776750.

Sanchez JLCooper MJ, Myers CA, Cummings JF, Vest KG, Russell KL, Sanchez JL, Hiser MJ, Gaydos CA.2015.Respiratory Infections in the U.S. Military: Recent Experience and Control. Clin Microbiol Rev 28:.https://doi.org/10.1128/cmr.00039-14

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