Infectious diseases Internal Medicine


H1N1 Influenza
H1N1 influenza virus particles.

Introduction – Influenza

Every winter we experience influenza epidemics. Influenza epidemics are characterized by increased mortality, especially in people over the age of 65 and people with cardiac and pulmonary disease.

There are three types of influenza viruses that infect humans, A, B and C. Influenza A causes the biggest epidemics. Influenza A can be divided into different subtypes depending on variants of the surface antigens, hemagglutinin (H) and neuraminidase (N). Influenza B also gives typical flu symptoms. The Influenza C virus causes only a mild cold. In addition, there are influenza types that affect specific species, such as the bird flu (Avian Influenza). In rare cases, infections from birds to humans can occur in highly virulent virus strains.

Influenza activity is still quite low so far in early January, but is expected to increase in late January and early February. So far, the strain H1N1 dominates, ie the same strain that in 2009-2010 was called the swine influenza.

Symptoms – Influenza

Symptoms of influenza A and B:

  • High fever
  • Headache
  • Muscle pain
  • Dry cough
  • Retrosternal Pain
  • Malaise
  • Runny nose
  • Redness of the conjuctiva (conjunctivitis)

Incubation time 1-4 days.

The duration of illness is usually 7-10 days, of which fever lasts during 3-5 days.

There are variations in time regarding how long patients are contagious. The contagiousness is greatest during the first few days. As long as a patient has fever that is correctly assessed to be due to influenza and not due to secondary complications, there is a risk of transmitting the virus.


  • Sinusitis, otitis
  • Pneumonia
    Often debuts after several days of the influenza onset. Antibiotics that have an effect on both pneumococci and staphylococci are recommended in severely ill patients with pneumonia associated with influenza.
  • Influenza Pneumonitis
    Often debuts early in the course of the disease. Causes problems with oxygenation of the blood.

Differential Diagnoses

  • Bacterial respiratory tract infections
  • Other respiratory viruses, for example:
    – RS virus
    – Rhino and coronavirus
    – Parainfluenza
    – Adenovirus.

Diagnostics – Influenza

At the beginning of an influenza outbreak, it is important to quickly determine the correct diagnosis and identify the influenza strain.

Swab sampling from the nasopharynx and pharynx for direct detection of virus by PCR are the most common routine methods. Occasionally, nasopharyngeal aspirate obtained by a fine catheter (e.g. baby feeding catheter) is taken after flushing with about 0.5 ml of saline for virus isolation.

Antibody tests may be of value in retrospect but are hardly useful in emergency situations.

Common laboratory tests such as CRP and WBC are often normal or slightly elevated in mild influenza. In severe influenza, CRP can rise to 50-100 mg / L, which can make differential diagnosis against bacterial infections difficult.

Prophylaxis – Influenza


Through vaccination, complications and deaths related to influenza can be reduced. Influenza vaccines consist of surface structures (neuraminidase and hemagglutinin) purified from viral cultures. The virus strains from which the vaccine is to be produced are determined annually by the WHO.

The vaccine’s protective effect is about 60-70% and this is reached about 2 weeks after vaccination and lasts about 6-12 months.

The following vaccines against seasonal influenza with purified antigen are commonly used:

  • Influvac
  • Vaxigrip

In addition, there is a live attenuated vaccine given as nasal spray (Fluenz) to children who are 24 months up to 18 years of age (should not be given to immunosuppressed or children treated with salicylates).

All vaccines contain antigens from influenza strains A and B as recommended by WHO. The vaccines are likely equivalent in efficacy.

The National Board of Health recommends seasonal influenza vaccination for the following patients:

All pregnant women are recommended to be vaccinated against seasonal influenza after pregnancy week 16 during the influenza period (autumn-winter). Vaccination is recommended even before pregnancy week 16.

Healthcare professionals who care for risk groups are also an important group for influenza vaccination. Vaccination prevents nosocomial infection.

Side effects

Local erythema and swelling at the vaccination site.
Mild fever reaction.


Pregnant women with underlying chronic disease may also be advised to get the vaccine.


Egg allergy. If you can eat eggs, vaccination can be proceeded with.

Drugs that stop virus reproduction

Antiviral drugs can sometimes be a complement, but do not replace vaccination. The most important area of ​​use for these agents is when the flu diagnosis is probable and there is a risk of complications (medical risk groups and pregnant women) as well as those who do not belong to medical risk groups but are so affected that they require hospital care.

Even influenza-exposed non-risk groups who wish to shorten the course of the disease may benefit from treatment early (preferably first-day symptoms). The earlier the treatment is applied, the better the effect. Anti-influenza drugs only have an effect on virus replication. After 48 hours of disease, viral replication rapidly decreases even in untreated subjects. In patients with severe illness requiring hospital care, antiviral therapy may also be initiated later in the course. This also applies to severely immunosuppressed patients.

Zanamivir (Relenza)

  • Selectively inhibits the formation of neuraminidase in influenza A and B.
  • The preparation is administered as an oral inhalation with 2 inhalations (2 x 5 mg) twice daily for 5 days.
  • Reduces the average illness time by 1-2.5 days (median 1.5 days)
  • No significant effect of the drug is seen later than 48 hours after disease onset.
  • The risk of complications in risk groups is reduced when treated with zanamivir.
  • Side effects of the drug are rare. One should be cautious in patients with severe asthma due to the risk of broncho-obstructive reactions. All asthma medications should be taken before zanamivir.
  • Zanamivir is active against avian influenza and new influenza.

Oseltamivir (Tamiflu)

  • Neuraminidase inhibitors that are effective against both influenza A and B, but with a relatively greater effect against influenza A in vitro.
  • Is registered for both treatment and prophylaxis.
  • Given per os. For treatment, give Tamiflu capsule 75 mg x 2 for 5 days to adults and children> 12 years. For prophylaxis give 75 mg x 1. For children 1-12 years, 2 mg/kg given twice daily (max 75 mg x 2) for 5 days as treatment and times 1 as prophylaxis.
  • On average, the illness time decreases by 1-1.5 days
  • There are some reports of nausea, vomiting, and abdominal pain. Tamiflu together with food reduces the risk of nausea and vomiting.
  • Tamiflu is active against bird flu and new influenza. However, an increased oseltamivir resistance in type A (H1N1) has been reported which requires careful monitoring of the resistance state.

Antiviral Prophylaxis

Recommended only for medical risk groups. In case of low risk of infection or less serious medical risk, early treatment is recommended for the first symptoms of influenza.

Further Reading