- Influenza (seasonal) is a highly contagious viral respiratory infection acquired through the inhalation of aerosolized respiratory droplets or direct contact with respiratory secretions from infected persons.
- Risk is high for travelers, depending on the destination and time of year, and for persons in close contact with infected persons.
- Symptoms include high fever, chills, severe body aches, headache, extreme tiredness, and dry cough.
- Consequences of infection may include pneumonia and respiratory failure. Death may occur, especially in those at extremes of age and in persons with underlying medical conditions.
- Prevention includes observing good respiratory-hygiene (cough and sneeze etiquette), hand-hygiene (frequent, thorough handwashing), and social distancing (maintaining a distance of 2 m [6 ft] from ill-appearing persons), especially during influenza season.
- Influenza vaccine is routinely given to all persons 6 months and older (including travelers) each year before the influenza season begins and may be required for destinations in which the influenza season is ongoing at the time of the trip. High-dose or adjuvanted (boosted) vaccines are recommended for persons aged 65 years and older. Vaccination reduces influenza incidence and thereby coinfection with coronavirus disease 2019 (COVID-19) which may increase the risk of a poor outcome. Persons acutely ill with COVID-19 should postpone influenza vaccination until after recovery.
- Vaccine side effects are most commonly injection-site reactions and, less frequently, fever and muscle aches.
- Duration of vaccine protection is about 6 months; no booster dose is recommended; however, revaccination should be considered by travelers who were vaccinated more than 6 months earlier if influenza risk exists at their destination (because of waning immunity).
"Classic" (seasonal) influenza is a highly contagious, viral infection, transmitted via inhalation of aerosolized respiratory droplets or direct contact with contaminated surfaces that causes significant illness and sometimes death. Influenza is transmitted in cool months in temperate climates and variably throughout the year in the tropics.
Avian influenza (bird flu) is a type of influenza limited to certain Asian and African regions that is fatal in birds but poorly adapted to humans. Transmission is through direct contact with live birds or their droppings or secretions and through consumption of undercooked eggs or poultry. Humans have no protection against bird flu, and no vaccines are commercially available for this infection. See Avian Influenza.
Seasonal influenza occurrence depends on the geographic area and time of year. Influenza season usually occurs during the winter months in the Northern and Southern Hemispheres but varies in tropical and subtropical countries. Some countries have specific seasonality, whereas others have influenza throughout the year, with periods of peak activity.
The 2019-20 influenza season in the US was the longest on record, likely due to increased visits for influenza-like illness that were actually coronavirus disease 2019 (COVID-19) occurring during the last 6 weeks of the season before COVID-19 testing became widespread.
Seasonal influenza viruses are transmitted from person to person via inhalation of aerosolized respiratory droplets (e.g., by coughing and sneezing), from close contact with an infected person, or contact with contaminated objects (e.g., touching surfaces contaminated with influenza virus and then touching the eyes, nose, or mouth).
Risk is high for travelers, depending on the destination and time of year.
Rates of infection are highest among children, but rates of serious illness, medical complications, and death are highest in children aged younger than 5 years, persons aged 65 years and older, and persons with underlying medical conditions that place them at increased risk for complications. Also at high risk are Native Americans and Alaskan Natives; blacks/African-Americans (due to increased risk of poor outcomes for COVID-19); pregnant, soon-to-be pregnant, and post-partum women; persons who are extremely obese; nursing home staff and residents; and persons with a weakened immune system.
On aircraft, a risk (very low) of infection exists for persons sitting within a 2-seat range (in back, front, and beside) of an infectious traveler.
Symptoms most commonly develop about 1 to 4 days (average: 2 days) following exposure and include high fever, severe body aches, chills, headache, extreme tiredness, and dry cough. Sore throat may occur. Influenza symptoms are usually more severe than cold symptoms (sore/scratchy throat, sneezing, runny/stuffy nose, and a mild cough) and are similar to those of COVID-19. With a cold, unlike with influenza, fever is rare in adults and older children, and severe body aches and extreme tiredness are not symptoms of a cold. Ill persons are contagious from 1 day before to up to 7 days after symptom onset (up to 10 days in children).
Avian influenza initially may be indistinguishable from seasonal influenza, presenting with fever and typical influenza-like symptoms, but rapidly deteriorates to lower lung disease (pneumonia), which can be fatal.
Consequences of Infection
Complications can include pneumonia and respiratory failure; inflammation of the heart, brain, or muscles; and worsening of underlying medical conditions (such as asthma or chronic heart disease). Death may occur in certain symptomatic cases.
Need for Medical Assistance
Persons who develop symptoms of influenza should seek immediate medical attention. Samples sent to public health laboratories for influenza testing will also be tested for influenza A, influenza B, and SARS-CoV-2; separate tests will need to be done for tests done in medical offices. Antiviral medications given with 48 hours of symptom onset may reduce the duration and severity of illness.
Observe respiratory-hygiene (cough and sneeze etiquette), hand-hygiene (frequent, thorough handwashing), and social distancing (maintaining a distance of 2 m [6 ft] from ill-appearing persons), especially during influenza season. Antiviral medications, an important adjunct to vaccination, are up to 90% effective in preventing influenza if started within 48 hours of first exposure.
A variety of influenza vaccines are available for the prevention of seasonal influenza. Vaccination is recommended yearly for all persons 6 months and older and is especially important for:
- Travel during the influenza season at the destination
- Travel to areas with current widespread influenza activity
- Travel to countries with a high intensity of influenza activity occurring in international gateways or highly visited destinations
- Cruise travel at any time of year
- Travelers in large organized tourist groups
- Travelers aged 50 years and older, children aged 6 months to 18 years, travelers of any age with chronic or immunocompromising conditions
- Persons in mass gatherings (e.g., Hajj and Umra pilgrims)
Vaccine effectiveness (greater in healthy adults aged younger than 65 years) depends on the match between the seasonal vaccine virus strains and currently circulating virus strains. High-dose (Fluzone High-Dose Quadrivalent) or adjuvanted (Fluad Quadrivalent) vaccines are recommended for persons aged 65 years, and older and every effort should be made to receive one of these vaccines; however, if not available, a recombinant vaccine (Flublok) should be used over a standard-dose inactivated vaccine.
Influenza vaccination is emphasized for the 2020-21 influenza season to reduce influenza incidence and thereby influenza symptoms that may be confused with those of COVID-19 and to reduce coinfection with COVID-19 which may increase the risk of a poor outcome.
The most common vaccine side effects are mild and include soreness at the injection site for up to 2 days. Less frequent side effects include fever, extreme tiredness, and muscle aches occurring 6 to 12 hours after vaccination and lasting for 1 to 2 days.
Persons with underlying medical conditions or who have concerns about the vaccine should speak to their health care provider before vaccine administration.
Influenza vaccine is given annually as follows:
- Persons aged 6 months through 8 years: 1 to 2 doses, depending on previous vaccination history. If 2 doses are necessary, both doses should be given before the end of October; 2 doses are still recommended even if the child turns 9 years between receipt of doses 1 and 2.
- Persons aged 9 years and older: 1 dose
Duration of vaccine protection is about 6 months.
Influenza is seasonal, usually occurring from October through May (peak activity occurs in January or later) in the Northern Hemisphere (NH) and from April through September (peak activity occurs in April or later) in the Southern Hemisphere (SH). The best time to be vaccinated is during September and October in the NH and during April in the SH (if feasible) due to concerns over waning immunity in persons vaccinated 1 to 2 months before the season begins, in case a late influenza season extends 6 months beyond vaccination. However, influenza vaccine can be given anytime during the season. Travelers going to the opposite hemisphere or to the tropics who have not been vaccinated that season should be vaccinated prior to their trip. Travelers who are facing influenza risk at their destination should consider revaccination if they were vaccinated with the current formulation more than 6 months earlier. Persons acutely ill with COVID-19 should postpone influenza vaccination until after recovery.
Although vaccination is the best prevention against influenza, some travelers are not able to receive influenza vaccine, and, in some circumstances, the vaccine may not be well matched to the circulating strains.
Antivirals to Treat Influenza
Antivirals can be used by travelers who become ill with fever and influenza-like symptoms. Treatment begun within 48 hours of the onset of symptoms can shorten illness by 1.5 days and reduce the risk of complications from influenza. Self-use of antivirals is not a substitute for seeking immediate medical care, and travelers should make every effort to start treatment only under medical advice.
Travelers going to areas with high prevalence of malaria should remember that a fever could be caused by malaria, which is a medical emergency. Thus, travelers with flu-like symptoms should contact a health care provider before taking antivirals.
Antiviral agents which may be used for treatment (and less frequently prevention) of influenza include:
Oseltamivir (Tamiflu), the preferred drug, is taken as follows:
- Persons aged 18 years and older: 1 pill twice per day for 5 days.
- Children aged 2 weeks through 17 years: Based on weight.
Zanamivir (Relenza), an inhaled drug, is taken as follows:
- Persons aged 7 years and older: 2 inhalations (using provided device) twice per day (4 total inhalations) for 5 days.
Peramivir (Rapivab), an intravenous drug, is administered as follows:
- Persons aged 2 years and older: one-time dose administered over 15 to 30 minutes.
Baloxavir marboxil (Xofluza), an oral tablet, is taken as follows:
- Persons aged 12 years and older: One-time dose based on weight.
Side effects of oseltamivir, such as nausea or vomiting, may be lessened by taking the drug with food. Zanamivir is not recommended for use in individuals with asthma, chronic lung diseases, or living in a nursing home. Delirium or abnormal behavior may occur with these drugs. Diarrhea is the most frequently reported side effect with peravamir, and headache, nausea, vomiting, and bronchitis are reported with baloxavir marboxil.
Antivirals to Prevent Influenza
Antivirals are generally not recommended if more than 48 hours have elapsed since the first exposure to an infected person. Persons taking an antiviral who develop a febrile respiratory illness suggestive of influenza should seek medical care. Some travelers may be advised to carry oseltamivir or zanamivir to prevent influenza after exposure and, ideally, the antiviral should be started within 48 hours of exposure.
Oseltamivir is taken as follows:
- Persons aged 13 years and older: 1 pill once per day for at least 7 days (up to 6 weeks during a community outbreak).
- Children aged 1-12 years: Based on weight.
Zanamivir is taken as follows:
- Persons aged 5 years and older: 2 inhalations once per day for 7 days (up to 28 days in a community outbreak).