Influenza (also known as the flu) is a contagious respiratory illness caused by influenza viruses. There are 4 types of influenza viruses: A, B, C and D. Influenza B and C infect humans. Influenza D viruses primarily affect cattle, while influenza A viruses infect humans and animals like horses, pigs, cats, dogs, minks, domestic and wild birds, and wild aquatic mammals such as seals, dolphins, and whales.

Influenza is not the same as the common cold. Although infection with the flu will have symptoms similar to the common cold, influenza is way more dangerous than the common cold. With the flu, you experience a higher fever, muscle aches, fatigue, and you may even experience chills, sweats, vomiting or diarrhea.

When an infected person coughs or sneezes, the flu virus is present in the millions of tiny droplets out of the nose and mouth. Anyone who breathes in the droplets can get infected with the flu, including healthy people, and complications related to the flu can happen at any age. Those at a high risk of developing complications include pregnant women, the very young (less than 2 years old), older adults (over 65 years of age), and people with chronic medical conditions such as diabetes, heart problems, asthma, chronic obstructive pulmonary diseases, cancer, HIV, and other conditions that impair or weaken the immune system.

Complications of flu are often seen in high-risk groups. The flu may cause bronchitis (inflammation of the bronchi), pneumonia (lung infection), ear infections, myocarditis (inflammation of the heart), encephalitis (inflammation of the brain) or myositis (inflammation of the muscles), and multi-organ failure. Although it is challenging to determine the exact number of deaths from the flu, the World Health Organization (WHO) estimates that influenza epidemics result in about 3 to 5 million cases of severe illness, and about 250,000 to 500,000 deaths. This is most probably a very conservative estimate because deaths due to pneumonia, stroke or heart attack that were triggered by a flu infection are not recorded as flu deaths.

To prevent flu infection, people are advised to follow good health habits like staying away from people who are sick, washing their hands frequently, covering coughs and sneezes, and staying at home if they feel sick to slow down the spread of the virus. But, the most important step in preventing the flu is to get a flu shot each year.

Influenza it frequently mutates until it is slightly different from the flu strains that have circulated in the past flu seasons. This is called antigenic drift, the changing of the surface structure of those flu strains – that reduce or even destroy, the effectiveness of the previous year’s vaccine. Antigenic drift can happen in all three types of influenza: A, B, and C. The WHO and the Global Influenza Surveillance and Response System (GISRS), collect data all year-round to determine which strains are circulating and to check how different the currently circulating strains are from the previous years. Influenza surveillance systems are in place all over the world even in developing countries in Asia and Africa. Together, the experts predict which strains will affect us this coming year and grow them ready for vaccines. It takes about six more months to develop and manufacture vaccines for chosen strains. There are flu seasons where scientists get it wrong, or not necessarily wrong, but genetic drift during the six months production time can render the newly formulated vaccine not as effective as it should be.

Occasionally, the virus might combine with another strain making it completely new. This happens when flu viruses co-infect susceptible mammalian hosts (like humans, birds, pigs), increasing the likelihood of mixing of genetic material. This recombination is called antigenic shift. Scientists are only starting to understand the mechanisms underlying the process, including the change in the efficiency of human-to-human transmission, which is a major factor in turning a flu strain into a pandemic one. Click here for more information on antigenic drift and antigenic shift.

Just like all other medications, the flu vaccine carries a small risk of adverse events, like redness, swelling, and soreness at the injection site which will resolve on its own after a few days. Sometimes a person may feel tired and achy after a flu shot, but this does not necessarily mean they have a flu infection. It is the body triggering an immune response to the vaccination. This is how vaccines "train" the immune system to recognize parts of the virus so it can respond quicker and more effectively when it encounters the real virus.

Effectiveness of flu vaccines can vary from year-to-year among different age and risk groups and even by vaccine type. Currently, available vaccines are generally 60-80% effective in children and healthy adults under the age of 65. In people older than 65 years of age, a lower rate of 30-50 % is seen - the number may seem low, but it is still significant in reducing the risk of influenza and its complications in the elderly.

Even if you are vaccinated against the flu and still contract the virus, the vaccine can help reduce the symptoms and duration of the illness. If you haven’t gotten your flu shot this year, go and visit your doctor or pharmacist. After being vaccinated, the body responds by producing antibodies. Protection starts about 2 weeks after vaccination so remember to get the flu vaccine at least 2 weeks before the flu season.

Research is ongoing to create a universal flu vaccine, one that would do away with getting a flu shot every year. But in the bigger picture of reducing flu transmission, and what we have presently, getting more people to get existing annual flu shots is the best way to reduce the thousands of deaths it causes worldwide and the risk of it spreading through the community.

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