The clinical spectrum of influenza ranges from asymptomatic infection to primary viral pneumonia that may progress to death.
Patients presenting with influenza-like illness (ie temperature of 37.8ºC, cough and/or sore throat and absence of a known cause other than influenza) might be infected with different types of influenza virus [eg avian influenza (H5N1)] as well as other respiratory pathogens.
A high index of suspicion is needed to recognize influenza in hospitalized patients.
Pneumonia is the most common complication of influenza virus.

Influenza Management


Follow local program for control of priority diseases, if available

Timing of Seasonal Influenza Vaccination

  • Influenza vaccine is administered yearly to provide optimal protection against influenza virus infection
  • Vaccine should be administered before influenza activity in the community begins
  • Vaccination should commence throughout the flu season due to its varying duration
  • Vaccination campaigns may start earlier (July-August or as soon as vaccines are available) to ensure that there is sufficient time for the community to be vaccinated against influenza while adapting to the social distancing practices implemented in different countries to reduce the spread of SARS-CoV-2

Northern Hemisphere

  • Typically administer annual vaccinations to high-risk individuals and their close contacts by the end of October 
  • Vaccination given in December or later might be considered in most influenza seasons
  • Influenza activity typically occurs during October-May

Southern Hemisphere

  • Typically administer annual vaccinations to high-risk individuals and their close contacts between March-May
  • Influenza activity typically occurs during April-September

Tropical or Subtropical Regions

  • Lab-confirmed influenza can occur anytime throughout the year
    • Peaks of influenza activity can occur 1-2 x/year
  • Epidemics often coincide with the rainy season
  • Public health programs where high-risk individuals and their close contacts are vaccinated should be performed at the same time each year if >1 peak of influenza activity occur within a year
    • Latest available vaccine formulation should be used
Influenza Vaccines

The following recommendations apply to seasonal influenza only; vaccines against avian influenza are still being developed

  • Vaccines are the primary preventive measure against influenza
  • Vaccine used should comply with current WHO recommendations
  • Influenza vaccine should be given annually for persons ≥6 months with no contraindication/s to any of its components
    • An age-appropriate formulation of inactivated influenza vaccine (IIVs), recombinant influenza vaccine (RIV) or live attenuated influenza vaccine (LAIV) should be given
  • Special populations that may be given influenza vaccine:
    • With history of egg allergy who only have hives after exposure
    • With history of egg allergy other than hives (ie angioedema, lightheadedness, recurrent emesis, respiratory distress and those who required epinephrine or other emergency medical intervention)
    • Pregnant women and for those who want to become pregnant
  •  Patients with egg allergy should be observed for 15 minutes to reduce the risk of injury in patients who experience syncope
  • Patients with a history of egg allergy of any severity may receive any licensed, recommended and age-appropriate IIV, RIV4 or LAIV4 influenza vaccine


  • For the 2020/2021 northern hemisphere flu season, the recommended egg-based trivalent vaccine should contain an A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus, an A/Hong Kong/2671/2019(H3N2)-like virus and a B/Washington/02/2019 (B/Victoria lineage)-like virus
    • Quadrivalent vaccines will contain above 3 viruses, plus a B/Phuket/3073/2013–like virus (B/Yamagata lineage)
  • For the 2020/2021 northern hemisphere flu season, the recommended cell culture- or recombinant-based trivalent vaccine should contain an A/Hawaii/70/2019 (H1N1)pdm09-like virus, an A/Hong Kong/45/2019(H3N2)-like virus, and a B/Washington/02/2019 (B/Victoria lineage)-like virus
    • Quadrivalent vaccines will contain above 3 viruses, plus a B/Phuket/3073/2013–like virus (B/Yamagata lineage)
  • For the 2020 southern hemisphere flu season, the recommended trivalent vaccine should contain an A/Brisbane/02/2018 (H1N1)pdm09-like virus, an A/South Australia/34/2019 (H3N2)-like virus, and a B/Washington/02/2019-like (B/Victoria lineage) virus
    • Quadrivalent vaccines will contain above 3 viruses, plus a B/Phuket/3073/2013–like virus (B/Yamagata/16/88 lineage)s
  • LAIV4 is an option for patients in whom it is appropriate

Trivalent Flu Vaccines

  • Contains 2 type A strains and 1 type B strains of influenza 
  • Standard-dose trivalent shots (IIV3) are manufactured using virus grown eggs and is recommended for 18-64 years
  • High-dose trivalent shot is approved for people ≥65 years of age
  • Trivalent shot with adjuvant is approved for people ≥65 years of age

Quadrivalent Flu Vaccines

  • Contains 2 type A strains and 2 types B strains of influenza 
  • Intradermal doses are approved for people 18-64 years of age
  • Two new quadrivalent vaccines (high-dose and standard dose with adjuvant) were recently approved by the United States Food and Drug Administration for patients ≥65 years of age
  • Quadrivalent flu shot with virus grown in cell culture is recommended for children ≥4 years of age

Inactivated Influenza Vaccine (IIV)

  • Given IM and contains killed or inactivated viruses so it cannot cause influenza
  • Licensed for use among persons aged >6 months, including those who are healthy and those with chronic medical conditions
  • Effects: If the vaccine used contains the predominant circulating influenza strains, it can be 70-90% effective in preventing illness in healthy adults aged <65 years
    • Approximately 50-77% effective when the vaccine strains were antigenically dissimilar to the majority of circulating strains
  • Patients with concurrent medical conditions may have a reduced rate of severe respiratory illness and death (up to 50%)
    • Main benefit of vaccination in these patients is the prevention of severe consequences of infection rather than preventing uncomplicated illness
  • Vaccines that conform to international standards of purity and potency are usually free from systemic side effects

Live Attenuated Influenza Vaccine (LAIV)

  • Given intranasally and contains live attenuated viruses that may cause mild signs and symptoms
  • Licensed for use among nonpregnant persons aged 2-49 years including healthcare personnel and other close contacts of high-risk persons (except severely immunocompromised persons who require care in a protected environment)
  • Recommended for healthy children 2-8 years with no contraindications or precautions
  • Effects: Significantly reduces the incidence of severe febrile illnesses, number of sick days, frequency of physician visits and antibiotic use
    • A 55% reduction in culture-confirmed influenza among children who received LAIV compared with those who received IIV
    • A 52% greater efficacy than TIV in preventing influenza among children aged 6-71 months who had previously experienced recurrent respiratory tract infection
    • A 32% increased protection in preventing culture-confirmed influenza in children and adolescents aged 6-17 years with asthma
  • LAIV should not be used in the following populations:
    • Children and adolescents who are receiving Aspirin or Aspirin-containing products
    • Persons who have experienced severe allergic reactions to the vaccine or any of its components and previous dose of any influenza vaccine
    • Pregnant women
    • Immunosuppressed children and adults (includes suppression caused by medication and HIV)
    • Caregivers and close contacts of immunocompromised individuals who require a protected environment
    • Children 2-4 years with asthma or with wheezing episode
    • Persons who took influenza antiviral medications within the previous 48 hours

Recombinant Influenza Vaccine (RIV)

  • Recently approved influenza vaccine that is neither inactivated or weakened
  • Recommended for patients aged ≥18 years allergic to egg protein with no other contraindications
  • Does not contain any egg protein
  • Uses recombinant DNA technology and insect virus expression system to produce an egg protein-less vaccine against seasonal influenza

Recommended Target Groups for Seasonal Influenza Vaccination

For individual protection, administration of seasonal influenza vaccination is recommended for the following persons at higher risk for complications secondary to severe influenza:

  • All persons aged ≥6 months without contraindications 
  • Children aged 6-59 months
  • Adults aged ≥50 years
  • Those who are immunocompromised, including that is caused by human immunodeficiency virus (HIV) and medications
  • Individuals with chronic pulmonary (including asthma, chronic obstructive pulmonary disorder, cystic fibrosis), cardiovascular (congenital heart disease, congestive heart failure, coronary artery disease except isolated hypertension), renal, hepatic, hematologic (including sickle cell anemia) metabolic disorders (including diabetes mellitus)
  • Individuals who are morbidly obese [i.e body mass index (BMI) ≥40]
  • Children and adolescents 6 months to 18 years of age currently receiving Aspirin- or salicylate-containing medications who will be at increased risk for Reye syndrome after a bout of flu
  • Women anticipating pregnancy and pregnant women during the flu season
    • The World Health Organization (WHO) considers the vaccine safe at any gestational age of pregnancy
  • Poultry workers, pig farmers and those in pig-slaughtering industry
  • Caregivers or other individuals who live with high-risk individuals
    • Healthy household members and caregivers of high-risk individuals
    • Healthcare workers
  • Residents of institutions for the physically and mentally disabled and other long-term care facilities
  • Others
    • Members of other groups are advised to consult their physician should they wish to obtain seasonal influenza vaccine


Seasonal Influenza 

  • Antivirals may be used to prevent spread of influenza during an influenza outbreak in an institutionalized setting
  • May be used in persons who cannot be vaccinated due to contraindications to vaccine
    • Do not start until influenza epidemic has begun and stop only when epidemic is over
  • May be used in persons at high-risk for complications who have been vaccinated after an outbreak of influenza has begun
    • Give for 2 hourly starting from day of vaccination to give sufficient time for immunity to develop

Avian Influenza

  • Where neuraminidase inhibitors are available:
    • In high-risk exposure groups, including pregnant women, Oseltamivir should be administered as chemoprophylaxis continuing for 7-10 days after the last exposure; Zanamivir could be used in thesame way as an alternative
    • In moderate-risk exposure groups, including pregnant women, Oseltamivir might be administered as chemoprophylaxis continuing for 7-10 days after the last exposure; Zanamivir might be used in the same way
    • In low-risk exposure groups, Oseltamivir or Zanamivir should probably not be administered for chemoprophylaxis
    • Pregnant women in the low-risk group should not receive Oseltamivir or Zanamivir for chemoprophylaxis
    • Amantadine or Rimantadine should not be administered as chemoprophylaxis
  • Where neuraminidase inhibitors are not available:
    • In high- or moderate-risk exposure groups, Amantadine or Rimantadine might be administered for chemoprophylaxis if local surveillance data show that the virus is known or likely to be susceptible to these drugs
    • In low-risk exposure groups, Amantadine and Rimantadine should not be administered for chemoprophylaxis
    • In pregnant women, Amantadine and Rimantadine should not be administered for chemoprophylaxis
    • In the elderly, people with impaired renal function and individuals receiving neuropsychiatric medication or with neuropsychiatric or seizure disorders, Amantadine should not be administered for chemoprophylaxis

Infection Control/Isolation Procedures

Infection Control Measures for Outpatients

  • Frequent hand washing with soap and water
  • Cover nose and mouth when sneezing or coughing
  • Dispose of nasal and mouth discharge and used tissue papers properly
  • Use of surgical mask by the ill person; paper mask is not recommended
  • All members of the household should be educated on personal hygiene and infection control measures, eg hand washing, use of surgical mask
  • Avoid sharing of utensils, towelsand bedding between patients and others
    • Clean all environmental surfaces soiled by body fluids with a household disinfectant according to manufacturer’s instruction, wear gloves during this activity
  • Adults: Restriction of social contacts (patient should not go to work, school or other public areas)
    • Continue infection control precautions until 21 days after resolution of fever and improved or absent respiratory symptoms
  • Children <12 years: Restriction of social contacts (patient should not go to school or other public areas)
    • Continue infection control precautions until 21 days after resolution of fever and improved or absent respiratory symptoms
  • Household members or other close contacts of diagnosed avian influenza patients who develop fever or respiratory symptoms should immediately contact local public health authority or WHO hotlines

Isolation Precautions for Hospitalized Patients

Isolation precautions should be implemented for all hospitalized patients with confirmed or suspected influenza A as follows:

  • Standard Precautions: Scrupulous hand hygiene before and after patient contact along with appropriate use of gloves/gowns when needed
  • Droplet Precautions: Wear a surgical mask if within 1 meter of the patient
  • Airborne Precautions: Controversial; warranted for selected patient care procedures
    • If available, place the patient in an airborne isolation room (ie monitored negative air pressure in relation to the surrounding areas with 6-12 air exchanges/hour)
    • When entering the room, use a high-efficiency mask, if available, or surgical mask


  • If a single room is not available, place patients separately in designated multi-bed rooms or wards
    • Beds should be placed 1 meter apart from other beds and preferably separated by a physical barrier (eg curtain or partition)
  • Limit the number of healthcare workers who have direct contact with the patient; these healthcare workers should not look after other patients
  • Restrict the number of visitors and provide them with the appropriate personal protective equipment (mask, gown, gloves and face shield or goggles) and instruct them in their use


  • Hospitalized patients discharged after <14 days should implement the necessary isolation precautions as listed above for outpatients


  • Clinicians should check with the local health authority for the latest updates because recommendations will change as new information becomes available

Follow Up


  • As resources permit, the local health facility should follow up non-hospitalized patients by home visits and telephone contact
  • Follow-up should be done within 24-48 hours as most avian influenza patients develop pneumonia on day 4-5 after onset of symptoms
  • Patient should seek medical treatment if condition worsens
  • Adults: Social contact restriction and infection control procedures for 7 days after resolution of fever and symptoms
  • Children <12 years: Social contact restriction and infection control procedures for 21 days after resolution of symptoms and fever


  • Isolation precautions should be continued for 14 days after the onset of symptoms until an alternative diagnosis is established or until diagnostic test results indicate that patient is not infected with influenza A virus
  • Serial respiratory and blood specimens to check for possible bacterial infection
  • For patients discharged before 14 days, it is recommended that they be isolated in the home setting
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