What is “Seasonal Flu” ?
- Seasonal flu is the “normal” flu that occurs every winter in both the Northern and Southern Hemispheres.
- In the Northern Hemisphere, peak transmission runs from December to April – roughly from Christmas to Easter, which is why vaccination programmes typically run from September to December.
- In the Southern Hemisphere, the season runs approximately from June to September.
- And in tropical countries, where there are no seasons, flu is present all-year-round – a hazard that visitors are not always aware of.
- Seasonal flu typically affects between 10 to 15 per cent of the population each winter.
Seasonal Flu Outbreaks and Epidemics
Superimposed on this annual seasonal pattern, larger outbreaks (epidemics) of seasonal flu can also occur – typically every 3-10 years. We have been fortunate not to have experienced an epidemic in recent years, but experts believe that another epidemic of seasonal flu is overdue and that the risk of a further epidemic rises significantly as each winter passes. (This is not the same as pandemic flu.)
It is impossible to predict how severe an epidemic of seasonal flu might be. Its severity depends on three factors: declining immunity in the population, how much the virus “mutates” in any given year, and the ability of a new virus strain to cause severe infection.
Flu is an infection of the lungs and upper respiratory tract caused by the flu virus. There are three main types of flu virus – called A, B and C. Only A and B cause significant human illness.
- Fever, typically with sudden onset
- Feeling unwell
- “Achiness” – particularly of the muscles and skin
- Loss of appetite
Symptoms are usually at their worst after 2-3 days. Recovery typically takes 5-8 days, though some people take 2 weeks or even longer to feel completely well again.
Although flu is often thought of as a minor illness, it is a common experience for people who are otherwise fit and healthy to be out of action for two weeks or longer.
Some people are more likely to become infected, and may also be more vulnerable to serious complications. Many of these people – e.g. the elderly, and people who are diabetic, asthmatic, have kidney problems or reduced immunity are prioritised for free flu vaccination under the NHS.
In such people, pneumonia is an especially important complication, and is usually the result of bacterial infection “taking advantage” of a weakened host.
How it is spread
The virus spreads from person to person in tiny droplets of mucus coughed or sneezed into the air by an infected person. These droplets dry out to form particles called droplet nuclei, that are easily inhaled by another person, to cause infection.
The incubation period – the time it takes from first contact with the virus until symptoms appear – is between 1 and 4 days (typically 2 days). It is usually possible to pass on the infection for as long as 24 hours before symptoms appear – one reason why the infection spreads so easily.
There are many upper respiratory viruses, with a range of severity, that can cause similar symptoms.The technology for detecting upper respiratory viruses has improved dramatically. Sensitive molecular testing is now available at the Fleet Street Clinic, that can identify flu and other important causes of upper respiratory infection within 65 minutes of taking a sample.
We are still used to thinking of flu as an untreatable viral infection that needs to run its course, but early diagnosis and treatment with antiviral medication can shorten the illness, reduce its severity, and reduce spread to close contacts and family members.