There are a number of excellent sources for information about the science of influenza. Basic information about the virus, symptoms, transmission and virulence, seasonality, and the influenza vaccine is provided here. For additional information about influenza, please visit the CDC and WHO websites.
Influenza, or “flu,” is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness and at times, can even lead to death. On a global scale it is estimated that seasonal influenza may result in 3-5 million cases of severe illness, and between 250,000-500,000 deaths each year. These estimates, however, are based on data from the United States and other industrialized countries. Less information is available, and therefore less is known, about the impact of flu in developing countries. Nevertheless, the information that does exist suggests that the risk of severe illness from influenza is likely higher in developing countries than in developed countries.
There are three types of flu viruses: A, B, and C. Type C only causes mild respiratory illness and is not thought to cause epidemics. Type C is not included in vaccines for those reasons. Types A and B, on the other hand, are what cause seasonal flu, and as such, are what vaccine manufacturers focus on when creating each season’s flu vaccine. More information on how flu virus strains are classified can be found on the CDC website. The image on the left is a 3D graphical representation of the biology and structure of a generic influenza virus (credit: CDC).
Some of the more common signs and symptoms of the influenza are:
- Fever or chills
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
Occasionally vomiting and diarrhea are also symptoms of influenza, although these tend to be more common in children than adults. In some severe cases influenza can lead to bacterial pneumonia, ear infections, sinus infections, and dehydration. It can also worsen existing medical conditions like asthma, heart problems, and diabetes. The best way to reduce the risk of experiencing these complications is to get vaccinated every year.
Transmission and Virulence
Flu spreads when a person who has the virus coughs, sneezes, or even talks and in doing so spreads droplets containing the virus. The droplets might land on another person’s mouth or nose or be breathed in, allowing the virus to find its way in to a new person’s body. It is also possible for a person to get the flu by touching something that flu droplets have landed on.
While practicing good hygiene habits, like frequent hand washing and not sharing utensils or food with someone who is sick, are good ways to avoid getting the flu, ultimately the best protection is to pair those habits with an annual influenza vaccination.
The flu is unpredictable and its severity and characteristics can vary widely from one season to the next. It all depends on several factors:
- Which flu viruses are spreading,
- How much flu vaccine is available,
- When the flu vaccine becomes available,
- How many people are vaccinated, and
- How well the vaccine matches the circulating viruses.
Influenza can vary depending on when and where it occurs. People living in temperate climates are most affected by influenza during the same general timeframe each year, the “flu season.” In the Northern Hemisphere, the flu season runs from October to March and is usually most active during December and January. In the Southern Hemisphere, the flu season runs from April through September, and is usually most active during June and July. In tropical areas, however, the flu may circulate year round, without a particular seasonality to it.
Not only are the seasons for the Northern and Southern Hemispheres different, often the dominant, circulating flu viruses in the two hemispheres are different. As a result, the vaccines for the two hemispheres are rarely the same. Prior to the start of each flu season, the WHO convenes a meeting and determines, based on existing surveillance, which viruses are likely to be the biggest circulators in the coming season. This decision then guides which viruses vaccine manufacturers will incorporate into the vaccine. For the Southern Hemisphere, the WHO makes its decision in the September preceding the start of that hemisphere’s flu season. For the Northern Hemisphere, the decision is made in the February prior to that hemisphere’s flu season. That gives manufacturers only a few short months to create and distribute a relevant vaccine for the upcoming season.
The flu vaccine is a biological preparation that induces immunity against influenza. The flu vaccine – often called a flu shot – is a combination of either three different influenza viruses (trivalent) or four different influenza viruses (quadrivalent). The trivalent vaccine protects against two influenza A viruses and one influenza B virus. It is administered via a needle into either the muscle or the skin. Some trivalent shots are grown in eggs and some are grown in a cell culture. The quadrivalent vaccine protects against two influenza A viruses and two influenza B viruses. It is administered either via a needle or a nasal spray. The flu shot cannot give a person the flu.
The flu vaccine is unique among vaccines since it has only a 12-month shelf life. The seasonality of flu and the short shelf life of the vaccine are notable challenges that PIVI deals with in securing and distributing seasonal flu vaccines to its partner countries.
Since the flu viruses that circulate change on a yearly basis, information about the vaccine changes just as frequently. The most up to date information about the recommended flu vaccine that is available each season as well as flu in general can be found on the CDC and WHO influenza websites.