Acute respiratory infections (ARI): Influenza (avian and seasonal)

Respiratory tract infections caused by influenza kill between 290,000 and 650,000 people every year (WHO data from 2017). Seasonal influenza is an acute respiratory infection (ARI) caused by influenza viruses that circulate in all parts of the world. An influenza epidemic occurs when an influenza A virus to which most humans have little or no existing immunity acquires the ability to cause sustained human-to-human transmission. This can lead to community-wide outbreaks. Such a virus has the potential to spread rapidly worldwide, causing a pandemic.

Avian and other zoonotic influenza infections in humans may cause disease ranging from mild infection of the eyes (conjunctivitis) to severe pneumonia and even death. Over the past several years, highly pathogenic avian influenza A(H5N1) has been detected in poultry, wild birds or other animals in over 30 countries, causing over 800 human deaths. Controlling the disease in the animal source is critical to decrease risk to humans.

The following are standard case definitions to allow national health authorities to interpret data in an international context. However, during an outbreak, case definitions may be adapted to the local context and the Red Cross Red Crescent should use those agreed/established by national health authorities. NB: Consider that during community-based surveillance, volunteers should use broad (simplified) case definitions (referred to as community case definitions) to recognize most or all possible cases, provide relevant risk communication and appropriate actions and encourage them to seek care. Other actors such as healthcare workers or investigators studying the cause of a disease, on the other hand, can use more specific case definitions that may require laboratory confirmation.

The main objective of influenza surveillance is not to identify every single influenza case, but instead to understand transmission patterns, disease burden and changes in the viruses that circulate globally. Accordingly, the following WHO-standardized case definitions are intended not to capture all cases but to describe trends over time.

Suspected case definition:

Influenza-like illness (ILI) case definition: An acute respiratory infection with measured fever of ≥ 38 C° and cough with onset within the last ten days.

Severe acute respiratory infection (SARI) case definition: An acute respiratory infection with history of fever or measured fever of ≥ 38 C° and cough, with onset within the last ten days that requires hospitalization.

Confirmed case definition: Patients meeting the ILI or SARI definitions who have laboratory confirmation of influenza virus infection.

WHO case definition source of information: https://www.who.int/bulletin/volumes/96/2/17-194514.pdf

There is no standard definition for an outbreak of seasonal influenza, which usually occurs in annual epidemics.

The term outbreak is more often used for the appearance of cases of a novel or emerging strain of influenza. A single case of a novel strain should trigger a public health response.

Risk factors for pandemic and seasonal influenza:

• No proper sanitation and hygiene measures preventing the droplet spread or the spread by hands contaminated with influenza viruses.
• Outbreaks of the disease are of particular concern when they occur in crowded settings including schools and nursing homes.
• No annual vaccination or when circulating viruses do not match the vaccine viruses.

Risk factors for avian influenza: direct or indirect exposure to infected live or dead poultry or contaminated environments, e.g. live bird markets. Other likely risk factors include slaughtering, defeathering, handling carcasses of infected poultry and preparing poultry for consumption.

Attack rates will vary from one outbreak to another. In case of an outbreak, consult the latest information provided by health authorities.

• It depends on the agent, context and affected population.
• About two-thirds of people with pandemic influenza are expected to develop clinical symptoms. Averaged over all (across all age groups), population clinical attack rates are expected to be 25–45 per cent.

• Elderly.
• Pregnant women.
• Infants under two months of age with signs of pneumonia/sepsis are at risk of suffering severe illness and death more quickly than older children.
• People with weakened immune system e.g. with sickle cell disease, asplenia, HIV, malignancy or receiving chemotherapy or steroids.
• Children younger than five years of age with chronic lung, liver or heart conditions.

Influenza is caused by infection with influenza viruses, RNA viruses of the Orthomyxovirus genus and are classified into types A, B, C and D. However, only types A and B commonly cause illness in humans. The four types co-circulate among humans around the world, but their distribution changes from one year to another and across geographic areas. The emergence of a new and very different influenza A virus with the ability to infect people and have sustained human to human transmission can cause an influenza pandemic. This is the reason why influenza A viruses are of most significance to public health.

Mainly only humans for seasonal influenza.

Zoonotic disease: Animals, particularly birds and poultry, for avian influenza.

• Seasonal influenza: Droplet and airborne spread: Large respiratory droplets when someone coughs or sneezes either by direct contact (e.g. touching hands), indirect contact (e.g. touching contaminated surfaces) or by airborne droplets.
• Avian influenza: Contact transmission: Human infections of avian influenza are acquired mostly through direct contact with infected animals or contaminated environments. Most avian influenza cases in humans of A(H5N1) and A(H7N9) are associated with direct or indirect contact with infected live or dead poultry. While recently identified avian influenza viruses do not currently transmit easily from person to person, the viruses have the potential to mutate to become more contagious between people.

Usually, two days for seasonal influenza but ranges from one to four days.

For avian influenza A(H5N1) virus infection in humans is about two to five days and ranging up to seventeen days. For infections with the avian influenza A(H7N9) virus, incubation averages five days, ranging from one to 10 days.

Depends on the infectious agent. This includes five days for influenza virus, but longer in infants, young children and people with compromised immune systems.

• Seasonal and avian influenza virus infections in humans may cause disease ranging from mild upper respiratory tract infection (fever and cough), to severe pneumonia, sepsis with shock and even death.
• Some common symptoms of pneumonia in children and infants include rapid or difficult breathing, cough, fever, chills, headaches, loss of appetite and wheezing.
• Children under five years of age with severe cases of pneumonia may struggle to breathe, with their chests moving in or retracting during inhalation.
• Young infants may suffer convulsions, unconsciousness, hypothermia, lethargy and feeding problems.
• Red eyes (conjunctivitis), gastrointestinal symptoms, encephalitis and encephalopathy have also been reported to varying degrees depending on subtype.

COVID-19 or MERS, respiratory syncytial virus (RSV), bacterial or mycotic pneumonia. Other similar symptoms exist for exacerbated chronic respiratory diseases and allergies.

• Conventional polymerase chain reaction (PCR) tests or real-time reverse transcription (RT-PCR) for the detection of influenza virus.
• Rapid influenza diagnostic tests (RIDTs) have lower sensitivity compared to PCR and their reliability depends largely on the conditions under which they are used.
• Viral antigen detection by immunofluorescence or enzyme immunoassay methods.

Please refer to the appropriate local or international guidelines for clinical management. All clinical management including the administration of any treatment or vaccine should be conducted by health professionals.

• Severe influenza disease might need hospitalization. Isolation is desirable.
• Antiviral drugs for influenza are available in some countries and may reduce severe complications and deaths.
• Vaccines against seasonal influenza are available. Because influenza viruses change rapidly due to antigenic drift, vaccines are reformulated and delivered annually, commonly through seasonal campaigns. Licensed influenza vaccines include inactivated or live-attenuated influenza type A and B viruses, with three or four subtypes per vaccine. Among adults, influenza vaccine provides protection or reduces severity of disease, even when circulating viruses do not exactly match the vaccine viruses. Inactivated influenza vaccines (IIV) are administered by injection; live-attenuated influenza vaccines (LAIV) are delivered as nasal spray. Only IIV’s are licensed for children younger than two years of age. Two doses of influenza vaccine given four weeks apart are recommended during the first season a child is vaccinated, followed by annual vaccination before influenza season. Maternal influenza immunization during pregnancy can protect infants too young to be vaccinated against influenza disease through transplacental transfer of antibodies.
• The World Health Organization (WHO) suggests that countries make decisions on influenza vaccines based on local disease burden, resources, capacity and other health priorities. The WHO recommends annual vaccination for pregnant women, children aged between six months and five years, elderly aged more than 65 years, individuals with chronic medical conditions and healthcare workers.

• Immunity following infection by one influenza virus will not fully protect against antigenic or genetic variants of influenza A or B viruses.
• Immunity from vaccination wanes over time so annual vaccination is recommended to protect against influenza. This is why new outbreaks occur every year and new vaccines have to be designed.