Influenza

 

     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 become widespread and result in training and operational disruptions. 

Seasonal vaccination is the most effective control measure available to reduce the risk of severe influenza and resultant mission degradation. Even when the seasonal vaccine is a suboptimal match to the circulating strain, as was seen last year with Subclade K, it generally retains an important degree of effectiveness against severe illness.

Policy and Guidance

Current DoW policy is that annual influenza vaccine is voluntary for DON Active Duty and Reserve Component members and DoW civilian personnel, with several exceptions:

• Initial entry training
• Healthcare personnel with direct patient contact or exposure to patients
• Deployed personnel with theater or country requirements
• Civilian personnel employed in child development and youth services programs
• Specifically designated forces

Policy Documents

• DoDI 6205.02 DoD Immunization Program
• DoW Memo updating DoDI 6205.02 for Reserve Component_29May2025
• DoW Memo: Updated Guidance on Influenza Vaccination_20Apr2026  
• USW (P&R) Department of Navy Annual Seasonal Influenza Vaccine Exception to Policy_17Jun2026 (*CAC Required*)
• USW (P&R) Reserve Component Seasonal Influenza Exception to Policy_24Jun2026 (*CAC Required*)
• USW (P&R) Force Health Protection Guidance-COVID-19 and Other Infectious Respiratory Diseases_26Jul2023
• NAVADMIN 177/26 2026-2027 NAVY INFLUENZA VACCINATION AND REPORTING POLICY
• MARADMIN 383/26
• NTRP 4-02.10 Shipboard Communicable Disease Prevention, Mitigation, and Response_Oct2021 (*CAC Required*)
• DHA Policy Memo 26-038_Guidance for Seasonal Influenza Vaccine_2Jul2026 (*CAC Required*)
• BUMEDINST 6230.15B Immunizations and Chemoprophylaxis for the Prevention of Infectious Diseases  
• BUMEDINST 6220.12D Medical Surveillance and Response

Web Resources

• DHA-IHD Influenza Vaccine Resource Center (*CAC Required*)
• CDC Influenza (Flu) Overview
• CDC Epidemiology and Prevention of Vaccine Preventable Diseases (“Pink Book”)
• CDC Manual for the Surveillance of Vaccine-Preventable Diseases
• ACIP Prevention and Control of Seasonal Influenza with Vaccines-2025-2026
• Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenza

Reporting

All Influenza outbreaks and influenza-associated hospitalizations of individuals under 65 years old are DoW Reportable Medical Events and must be documented in the Disease Reporting System internet (DRSi). Details may be found in the Armed Forces Reportable Medical Events Guide. In summary, for reporting purposes a confirmed influenza-associated hospitalized case is defined as a clinically compatible illness with BOTH of the following:
• Younger than 65 years of age and
• Any positive influenza laboratory test (example: culture, DFA, IFA, rapid, PCR)
AND
• Hospital admission date was ≤ 14 days after a positive influenza test or
• Hospital admission date was ≤ 3 days before a positive influenza test

The patient’s influenza immunization history should be included in the report 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. 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. Operational medical departments without ready access to the internet can report to their nearest MTF or NEPMU via phone, email, or message.

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. 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

The CDC routinely monitors influenza activity. In the U.S., peak influenza activity occurs between December and March, with February being the most common peak month. Because of the unique stressors and high-density living conditions in training environments and aboard ships, influenza outbreaks in the military occasionally occur outside the typical influenza season.

Ships and stations should monitor influenza trends in their supported populations and surrounding geographic locations 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 within a single unit or across 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.
Additional reports and tools for conducting routine medical surveillance can be found at the below links:

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

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 high levels of immunization among 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. 
In the event of an influenza outbreak, the following actions are recommended:

• Initiate enhanced clinical and public health surveillance for febrile respiratory illness. A line list should be prepared and populated with confirmed and suspected cases. Sample Line List
• Ensure appropriate clinical testing is being conducted. The lab should be advised to send several specimens for full virus characterization.
  •  DoD Global Respiratory Pathogen Surveillance Program (*CAC Required)
  •  Influenza Specimen Collection
• Institute non-pharmacologic interventions to reduce spread, including enhanced hygiene measures and isolation of ill personnel. Specific resources:
  • USW (P&R) Force Health Protection Guidance-Coronavirus Disease 2019 and Other Infections Respiratory Diseases
  • NTRP 4-02.10 Shipboard Communicable Disease Prevention, Mitigation, and Response_Oct2021 (*CAC Required*)
  • Fleet Respiratory Guide 
  • Army TG-314 Non-vaccine Recommendations to Prevent Acute Infectious Respiratory Disease
• Consider initiating a voluntary influenza vaccination drive early in the outbreak.  Acceleration of the regular influenza vaccination schedule may be indicated in training environments.
• 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. 
• Report outbreaks through DRSi (see Reporting).
• Contact local Preventive Medicine resources 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

Sayers DR, Ying S, Eick-Cost AA. Seasonal influenza hospitalization incidence rates among U.S. active component service members, 2010-2024. MSMR. 2026; 32(10):9-12. Published 2026 Jan 5.

Uyeki TM, Bernstein HH, Bradley JS, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenza. Clin Infect Dis. 2019; 68(6):895-902. doi:10.1093/cid/ciy874