With fall and colder weather practically at our doorstep, it's time to dive into an important matter of health: flu shots.

Doctors and pharmacists across Turkey have been calling on their at-risk patients to get vaccinated ahead of a flu season like no other. As if flu season wasn't deadly enough on its own – with the World Health Organization (WHO) estimating 650,000 people die annually from the infectious disease – this year we also have the added burden of dealing with another respiratory illness, COVID-19. Hence, to ease the strain on health care systems across the globe, it is all the more crucial that regular cases of flu and other seasonal diseases are kept to a minimum.

One way of ensuring that large populations do not end up hospitalized with the flu is to ensure effective immunity, and in this case, this is achieved by the influenza shot or flu shot, as it is more commonly known. Characterized by an infection in the nose, throat and sometimes lungs, the flu can cause mild to severe illness and sometimes lead to death in people with compromised immune systems. Expect to see symptoms such as a runny or stuffy nose, a sore throat, fatigue, phlegmy cough and, on occasion, a fever. (Check this guide to see if you have the flu or COVID-19.)

If you are infected with the influenza virus, the onset of symptoms usually comes in about one to four days, with the first three to four days being the most contagious, according to the Centers for Disease and Control and Protection (CDC). Flu is a rapidly spreading disease, and like the coronavirus, is transmitted via infectious droplets that are dispersed into the air up to 1 meter (3.3 feet) from coughs or sneezes. The best way to prevent the flu, science and data show, is by getting vaccinated.

How to prevent the flu?

Of course, having every single person vaccinated is neither plausible nor necessary. For most of the population to be protected, according to the U.S. National Foundation for Infectious Diseases, an immunization rate of around 50% is needed, a figure lower than most other vaccine-preventable diseases. For comparison, measles needs about 95% of the population to be vaccinated to create herd immunity.

The WHO closely follows the changes the influenza virus undergoes and makes annual recommendations for the vaccine. In essence, scientists and medical experts play a game of chess against the virus, trying to predict its next move, or in this case, its next mutation, and accordingly come up with a vaccine containing an attenuated or dead virus.

In recent years, flu vaccines have been trivalent, meaning that they contained the three most representative virus types in circulation, namely two subtypes of influenza A and a subtype of influenza B. Influenza virus A and B are the strains responsible for the seasonal epidemics we see every year. Influenza viruses also have two other types – C viruses are mild enough to not cause concerns for public health while virus D is impacts cattle and is not known to infect humans, according to the WHO.

Scientists try to predict which strain will be more prevalent in the year ahead, and if the vaccine they created has antigenic similarity with the viruses circulating among people that season, then the shot can provide 50%-80% protection for those who get inoculated. The level of protective antibodies in healthy adults who receive the vaccine are reported to be as high as 80%, though this figure does drop in the elderly. However, studies have proven that getting vaccinated reduces flu-related complications and deaths in the elderly and chronically ill.

Who should be vaccinated?

The Turkish Health Ministry recommends: