Influenza A is a contagious acute respiratory infection caused by two subtypes of viruses: H1N1 and H3N2. Type A and B viruses are responsible for seasonal influenza epidemics. Symptoms, causes, treatments, vaccine… Here is everything you need to know about influenza A with Professor Bruno Lina, virologist.
In Europe, 2 to 8 million people are affected by seasonal flu each year. This contagious acute respiratory infection is caused by the influenza virus which mainly affects the upper respiratory tract (nose, throat, bronchi), more rarely the lungs. There are two groups of viruses that can cause influenza in humans: the type A virus, the two subtypes of which are H1N1 and H3N2, and the type B/Victoria virus. “These three viruses co-circulate together and can cause an epidemic wave”, points out Professor Lina.
Only type A viruses have pandemic potential, recalls the Pasteur Institute on its site. Discover the symptoms, the diagnosis, the emergencies to consult your doctor, the treatment and the possible complications of this infection with our expert, Professor Bruno Lina, virologist with the Hospices Civils de Lyon.
What are the symptoms of influenza A?
Influenza A manifests itself by a set of symptoms called influenza syndrome:
- A sometimes very high fever that appears suddenly;
- Muscle and joint pain;
- Headaches ;
- ENT symptoms (cough, runny nose).
“There are also mildly symptomatic and asymptomatic forms.”, points out the virologist.
Be careful, flu syndrome can also be caused by viruses other than influenza: respiratory syncytial virus (RSV), rhinovirus (causing the common cold), parainfluenza virus or even adenovirus. If you have flu-like illness, it doesn’t necessarily mean you have the flu. The only way to know if you have the flu is to take a respiratory sample which will be analyzed in the laboratory.
Different symptoms in children
Children with the flu can suffer from the same symptoms as adults. However, some important differences should be noted:
- Newborns and infants may have unexplained high fever, that is to say without any other symptoms;
- In children, fever usually exceeds 39.5°C, which can trigger febrile convulsions;
- Young children with the flu may have a croup (an infection of the throat and vocal cords that causes a cough similar to a dog’s bark), pneumonia or a bronchiolitis;
- Young children may suffer from digestive disorders (stomach aches, vomiting, diarrhea), ear pain and have the Red eyes ;
- Some children may complain of significant pain in the legs or back linked to muscle swelling.
Note that the flu is potentially more serious in children under two years of age and in those with chronic illnesses.
How to diagnose influenza A?
And nasopharyngeal swab can diagnose influenza A.
It consists of introducing a kind of long cotton swab (the swab) into the nostril up to the nasopharynx to collect as many cells as possible by rotating the swab. This sample is then analyzed in the laboratory.
How is influenza A treated?
Treatment of influenza A is primarily symptomatic. THE paracetamol is recommended as a first-line treatment to relieve headaches, muscle and joint pain and to reduce fever. It is also necessary hydrate well (drink sweet herbal teas and broths) and rest.
Antiviral medications may be prescribed in some cases. The best known is the Tamiflu® (oseltamivir), active on type A and B viruses and available in oral form. To be effective, it must be given very early, that is to say within 48 hours of the start of symptoms. Taken early, it reduces the risk of complications and mortality. L’oseltamivir can also be given for prevention in certain situations, epidemics in EHPAD for example. In case of resistance to Tamiflu, another antiviral can be prescribed in the hospital intravenously: zanamivir (Relenza®).
“Taking anti-inflammatories should be avoided in case of flu because they can promote complications and secondary infections.”, warns Professor Lina.
How does the disease progress?
Symptoms appear 1 to 4 days after contamination. The virus can stay in the body for 4 to 5 days. The infection most often lasts a week but symptoms can last longer. In fact, fatigue and cough can persist for several weeks. “This is not due to the virus itself but to the body’s inflammatory reaction to the virus. To defend itself against the virus, our body will produce a ton of interferons, cells of the immune system responsible for coughs, fatigue and even aches.”, explains Professor Lina.
Some fragile people are more at risk of developing a serious form requiring hospitalization in intensive care or the use of ventilatory assistance. : people aged over 65, pregnant women, immunocompromised people, people suffering from a cardiac or respiratory pathology, the morbidly obese, infants and diabetics.
According to Santé Publique Europe, influenza epidemics represent on average 1 million consultations in community medicine, more than 20,000 hospitalizations and around 9,000 deaths each year in Europe.
How to prevent influenza A?
Disease prevention is based on personal protective health measures and vaccination.
Individual protection measures
Influenza viruses are easily transmitted by aerosol, via microdroplets and particles excreted when we speak, cough or sneeze. Transmission of the virus can also occur through the hands when we touch a contaminated surface and bring our hands to our mouth or nose. It is therefore essential to put in place barrier measures during epidemics:
- Wear a mask in public places or in the presence of sick people;
- Wash your hands regularly;
- Regularly clean objects and surfaces that may be contaminated;
- Cough and sneeze into the crook of the elbow;
- Ventilate your home regularly;
- Use disposable tissues;
- Avoid contact with vulnerable people if you are sick.
Annual flu vaccination remains the most effective way to protect yourself from the disease. The vaccine helps reduce severe forms of influenza. Vaccination is strongly recommended for the most vulnerable people mentioned above, for whom the vaccine is free. Family members of infants under 6 months of age at risk of serious influenza are also invited to be vaccinated. “Once vaccinated, it takes 15 days to 3 weeks to be protected from infection. Protection is optimal for 3 months. This is why it is preferable to get vaccinated before the epidemic arrives, ideally during the first days of November”, indicates the virologist. The seasonal flu vaccine can be prescribed by a doctor or midwife.
The influenza vaccines available in Europe are inactivated vaccines (which do not contain live infectious agents) composed of 3 or 4 strains. The composition of these vaccines is adjusted each year because influenza viruses undergo constant genetic changes. “Every year, vaccines are adapted to surveillance data on influenza viruses circulating in the world issued by the WHO”, indicates the site Vaccination-info-service.fr.
Type A influenza viruses also responsible for avian flu
Type A influenza viruses can also cause avian influenza, including subtypes H5, H7 and H9. This infection can affect almost all species of birds, whether wild or domestic. More rarely, avian flu can infect other animal species such as pigs and other mammals, including humans. Thus, the H5N1 virus, present in Asia, is pathogenic for humans. Transmission of this virus currently only occurs from animals to humans, but future transmission from humans to humans is not impossible. If this type of transmission were to happen one day, it could lead to a pandemic, such as that caused by the SARS-CoV-2 virus responsible for the disease covid-19.
Avian flu is most often asymptomatic in wild animals but is highly contagious and can cause extremely high mortality in industrial chicken, turkey and duck farms. This explains the systematic slaughter of chickens and other poultry in Europe if a case has been detected on the farm.