Influenza is a single-stranded RNA virus that is transmitted through inhalation of respiratory secretions. It causes seasonal flu in the winter time, and annual vaccinations are available.
🦠 Pathophysiology and serotypes
It is transmitted by respiratory droplets that may become aerosolised with coughing, sneezing or talking.
The RNA of influenza codes for 5 structural proteins and 3 non-structural proteins. There are 2 proteins M and NP which are used to classify the virus into its 3 serotypes based on antigenic differences.
- A - causes annual flu outbreaks in the winter. There are different H and N subtypes of influenza A which have been responsible for pandemics;
- H1N1 - swine flu.
- H5N1 - avian flu.
- B - causes an outbreak every 4 years usually, with milder disease than A. Usually in late autumn - early spring.
- C - causes mild disease and is not associated with pandemics or epidemic.
The reason we need annual vaccinations is because of antigenic drift - small point mutations of the virus that allow it to evade the immune system and cause infection.
Once infected, the virus enter the tracheobronchail epithelial cells using haemagglutinin antigen (H antigen) present on the cell surface. It then begins to replicate and lead infection. The neuraminidase antigen (N antigen) cleaves sialic acid to free the virus once replicated. Peak viral shedding occurs in the first 2-3 days of exposure. It usually resolves within 10 days.
⚠️ Risk factors
- Extremes of age - children <5 years and adults >65 years are most likely to have risks of serious complications.
- Immunocompromise
- Unvaccinated
- Healthcare workers
- CKD and diabetes
😷 Presentation
It usually has a more acute onset as compared to the common cold:
- Coryzal symptoms
- Rhinorrhoea
- Nasal congestion
- Sneezing
- Fever
- Lethargy and fatigue
- Anorexia
- Myalgia and arthralgia
- Headache
- Dry cough
- Sore throat
🔍 Investigations
It can be diagnosed clinically, but nasal or throat swabs can be taken for PCR testing. This can confirm infection and provide information for the Public Health Authorities.
🧰 Management
Healthy patients who are not at risk of complications do not require treatment and the infection will self-resolve.
Patients who are at risk of complication or already have complicated influenza may require some antivirals to be prescribed within 48 hours of symptoms.
At risk groups include:
- >65 years old or <6 years old
- Pregnant women
- Hyposplenism or asplenia
- COPD or asthma
- Chronic cardiac disease
- CKD
- Chronic neurological conditions
- Diabetes mellitus
- Immunosuppressed
- Morbid obesity (BMI >40)
Treatment options include:
- 🥇 Oral oseltamivir - 75mg 2x daily for 5 days.
- 🥈 Inhaled zanamivir - 10mg 2x daily for 5 days.
Post-exposure prophylaxis should be prescribed for at risk patients who have had close contact with influenza in the past 48 hours.
Prophylaxis options include:
- 🥇 Oral oseltamivir - 75mg 2x daily for 10 days.
- 🥈 Inhaled zanamivir - 10mg 2x daily for 10 days.
🚨 Complications
- Pulmonary
- Viral pnuemonia
- Secondary bacterial pnuemonia
- Worsening COPD/asthma
- Encephalitis
- Cardiovascular
- Mycoarditis
- Heart failure
- Febrile convulsions - in young children
💉 Vaccinations
Before 2013, vaccines were only given to elderly and at risk groups.
There are 2 types of vaccines, one is given to children and the other is for adults and at risk groups. Let’s take a look at both:
3 important aspects to children’s flu vaccines:
- Given intranasally - it is more effective than the injectable vaccine.
- First dose given at 2-3 years old and then annual vaccines given subsequently. Only children aged 2-9 who have not received a flu vaccine before need 2 doses.
- Live vaccine
🚧 Contraindications
- Immunocompromised
- <2 years old
- Current febrile illness
- Current wheeze
- Egg allergy
- Breastfeeding
- Aspirin therapy - for Kawasaki’s disease for example. Risk of Reye’s disease.
This is an inactivated vaccine and cannot cause influenza. The current vaccines are trivalent for 2 subtypes of influenza A and one subtype of influenza B. It is 75% effective (but decreases in elderly). It takes 10-14 days for antibodies to reach protective levels. It needs to be stored between 2-8ºC away from light.
Vaccination is recommended annually for all people >65 years old or for at risk individuals >6 months old.
Other at risk individuals who should be vaccinated include:
- Healthcare and social care workers
- Long-stay care home residents
- Carers of elderly or disabled
It may cause a fever and malaise for 1-2 days.
🚧 Contraindicated in patients with egg hypersensitivity.