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Seasonal Flu

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.

(In tropical countries, where there are no seasons, flu is present year-round - a hazard that visitors are not always aware of; in the Southern Hemisphere, the season runs approximately from June to September.)

Seasonal flu typically affects between 10 to 15 per cent of the population each winter. 

Outbreaks and epidemics

Superimposed on this annual 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.

The Illness

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.

Typical symptoms are:

  • Fever, typically with sudden onset
  • Feeling unwell
  • "Achiness" - particularly of the muscles
  • Loss of appetite
  • Cough

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 

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.

Prevention with Vaccines

Seasonal flu vaccines have a long track record of safe and effective use. The vaccine is normally given as early as possible in the autumn, and protection lasts through the winter and beyond, though because of mutation, new vaccines are  generally required each year.


Their effectiveness depends on there being a good match between the strains used to produce the vaccine, and the strains that ultimately cause infection. Seasonal flu vaccines cut the risk of infection by at least 70-90 per cent. Over time, our ability to predict which strains will arrive each year - based on global surveillance by the WHO - has dramatically improved. The effectiveness of vaccines in preventing flu is therefore constantly increasing.

Published research data show that seasonal flu vaccination:

  • Cuts the risk of laboratory-confirmed flu infection by at least 70-90%
  • Cuts total numbers of upper respiratory infections/flu-like illnesses* by at least 25-34%
  • Cuts work loss from upper respiratory infections/flu-like illnesses* by 32-43%
  • Cuts visits to the doctor by 42-44%

*Crude totals of colds, coughs, and all upper respiratory illnesses

Which type of vaccine?

There is a tendency to think that all flu vaccines are the same, but in fact there are a number of different types of vaccine currently available, with subtle differnces between them; there is also much ongoing research to create further options:

  • "Sub unit" or "surface antigen" vaccines contain killed flu virus surface proteins only; vaccination results in antibodies that are highly strain-specific to the vaccine viruses from which they are made.
  • "Split virus" or "split virion" vaccine contains fragments of whole, killed virus. Vaccination produces antibodies against strains that are an exact or very close match to the strains used to create the vaccine.
  • "Virosomal" vaccines contain the reconstituted virus membrane, including its outer cout, but without its core (which contains the viruses genetic material). Virosomal vaccines consist of virus-shaped and -sized particles. This means that they deliver killed viral particles directly inside target cells, stimulating broader and more cross-reactive protection ("cell mediated immunity") against flu viruses in general.
  • "Whole virus" vaccines contain the entire killed virus. This includes its inactivated core genetic material. This technology allows them to be made from pathogenic viruses rather than genetically modified or "lookalike" strains – hence their enhanced cross-protection. Whole virus vaccines are too pathogenic to grow using traditional egg-based methods (while growing, they kill the eggs), so have to be grown in cell culture - a promising technique for the future. 

The above technologies therefore make up a hierarchy of killed vaccines, that is decreasingly specific to the strains from which the vaccine is made, and that provides increasing cellular (T-cell, or CD4) immunity, and therefore potentially increased cross-protection.

  • Live attenuated vaccines (FLUENZ) which create local immunity in the upper respiratory tract, and are currently recommended for children.