1. What is the flu?

Flu is an acute, viral infectious disease of the respiratory system, caused by influenza viruses type A, B, and rarely C. It spreads:

by airborne droplets – during coughing, sneezing, speaking,
by direct or indirect contact – shaking hands, touching the same objects, and then the nose, mouth, or eyes.

The source of infection is a sick person – also one who does not yet have symptoms.

Incubation period: usually 1–4 days (most often 1–2 days).
Period of infectivity:
- adults: usually approx. 1 day before symptoms appear,
- children: up to 6 days before symptoms,
- people with severe immune deficiencies: can infect for many weeks or even months.

Who is most at risk?

children (especially <5 years old),
elderly people (especially ≥60–65 years old),
pregnant women,
people with chronic diseases (including heart, lung, kidney diseases, diabetes, cancer, reduced immunity),
people having frequent contact with others (health care workers, teachers, service workers, travelers).

2. Symptoms of the flu

Most common symptoms of the flu:

sudden onset of malaise,
fever above 38°C, chills,
muscle, joint, headache,
cough (most often dry),
sore throat, runny nose,
weakness, lack of appetite,
nausea and vomiting (more often in children).

How does the disease proceed?

Acute symptoms usually last 3–7 days,
Feeling of fatigue and weakness can persist for even more than 2 weeks.
In young and generally healthy people, the flu usually proceeds without complications, but can be very tiring.

Possible complications of the flu:

pneumonia and bronchiolitis,
otitis media (more often in children),
myocarditis or pericarditis,
exacerbation of chronic diseases (e.g., asthma, heart failure),
meningitis, encephalitis, myelitis,
secondary bacterial infections (including pneumococci, meningococci),
sepsis, respiratory failure.

When is urgent medical consultation / ER necessary?

Immediately contact a doctor or call for help if a person sick with the flu experiences, among others:

increasing shortness of breath, difficulty breathing,
pain or pressure in the chest,
high fever persisting >3 days or recurring after improvement,
disturbances of consciousness, confusion, convulsions,
bluing of lips, face, or fingers,
symptoms of dehydration (infrequent urination, strong thirst, dry mucous membranes, in children – lack of tears, sunken fontanelle),
in children: very apathetic behavior, breathing problems, reluctance to drink.

In the elderly, chronically ill, and small children, it is not worth delaying contact with a doctor, even when symptoms seem "typical".

3. Where does the flu occur?

Flu is a disease occurring all over the world.

Every year, WHO estimates even a billion infections worldwide.
In Poland, every season, from several hundred thousand to several million cases are recorded.

Seasonality:

In the temperate zone (Europe, large part of North America, part of Asia):
- the peak of cases falls in the winter period,
- in Poland, the most cases are usually between January and March.
In the intertropical zone (tropical):
- cases occur throughout the year, without a clear season.
In the southern hemisphere (e.g., Australia, Argentina):
- the flu season falls from April to September, i.e., the opposite of ours.

Information for travelers

The risk of getting the flu concerns every latitude. It is particularly important for:

people traveling in winter in Europe, North America, or Asia,
people going in the flu season to the southern hemisphere (e.g., to Australia, South America),
travelers with chronic diseases, seniors, pregnant women, children,
people using planes, cruise ships – close contact with others promotes infections.

4. How to protect yourself against the flu?

A. General prevention

It is impossible to completely eliminate the flu virus from the environment, but the risk of infection can be significantly reduced:

Hand hygiene
- frequent hand washing with soap and water,
- or disinfection with an alcohol-based preparation (especially after returning from transport, mall, school, work).
Cough and sneeze hygiene
- sneezing and coughing into a tissue or the bend of the elbow,
- immediately throwing tissues into the bin.
Avoiding close contact with the sick
- not hugging, not shaking hands with a person evidently having a cold / with an infection,
- staying at home when we are sick ourselves (protection of household members and coworkers).
We do not touch the face
- limiting touching the mouth, nose, eyes – this is the main "entry route" of the virus into the body.
Ventilating rooms, avoiding crowds, especially in the flu season,
Strengthening immunity: adequate amount of sleep, balanced diet, physical activity, avoiding smoking.

In the context of travel:

avoiding crowded spaces when possible,
frequent use of hand gel on the plane, at airports, in transport,
if recommended – using protective masks in places with increased risk.

B. Vaccination against the flu

Vaccination is the most important and effective way of flu prevention.

Type of vaccine

In Poland, mainly inactivated flu vaccines are used (containing "killed" viruses or their fragments – split/subunit):

every year they have an updated composition, matched to virus strains circulating in a given season,
usually contain antigens of 3 or 4 virus strains (trivalent or quadrivalent vaccines),
in vaccinated people, they reduce the risk of illness and – what is particularly important – significantly reduce the risk of severe course and complications.

For whom is vaccination recommended?

Every year, vaccination against the flu is recommended for:

people with chronic diseases (heart, lung, kidney, liver diseases, diabetes, cancer, immune deficiencies, after transplants),
people aged ≥55–60 years, and especially ≥65 years,
pregnant women and those planning pregnancy,
children from the age of 6 months,
health care and public service workers,
people having close contact with seniors and the chronically ill (family and professional),
people traveling in the flu season, especially from risk groups.

In practice – anyone who wants to reduce the risk of illness and complications can get vaccinated.

Vaccination schedule (generalized)

Children 6–35 months – 1 or 2 doses of 0.25 ml (depending on previous vaccinations).
Children 3–8 years – 1 or 2 doses of 0.5 ml.
Children from 9 years of age and adults – most often 1 dose of 0.5 ml.

If the child has not been vaccinated against the flu before – 2 doses are usually recommended at an interval of at least 4 weeks. If vaccinated in the previous season – 1 booster dose is usually sufficient.

The vaccine is administered intramuscularly or subcutaneously, most often in the arm.

Booster doses and duration of immunity

Immunity after vaccination develops within 2–3 weeks.
It usually lasts 6–12 months, which is why vaccination should be repeated every year – the vaccine composition changes, and immunity weakens.

Additional notes

Inactivated vaccines are not able to cause the flu because they do not contain live viruses capable of multiplying.
However, they can cause mild general symptoms (pain at the injection site, slight fever, malaise) – usually disappear spontaneously within 1–3 days.
There is also a nasal vaccine with live attenuated virus; however, it is not intended for everyone (e.g., not for pregnant women and people with immune disorders).

5. Summary

Flu is not a "common cold", but a serious disease that causes millions of cases and numerous complications every year – especially in the elderly, chronically ill, pregnant women, and small children.

Vaccination against the flu:

reduces the risk of illness,
significantly reduces the risk of severe course and complications (e.g., pneumonia, exacerbations of chronic diseases),
is particularly important before the winter season and before traveling in the flu season (also to the southern hemisphere).

It is best to get vaccinated about 2–3 weeks before the expected flu season or departure, so that the body has time to develop immunity.

If you are planning a trip, have chronic diseases, are pregnant, work with people, or simply want to reduce the risk of illness – it is worth talking to a doctor or pharmacist about vaccination against the flu.