Flu after vaccination: is it possible to get infected and what to do

Influenza is not just a "cold," as is often thought. It is an acute viral disease of the respiratory tract caused by RNA viruses of the *Orthomyxoviridae* family, predominantly types A and B. The influenza virus aggressively attacks the epithelium of the upper and lower respiratory tracts, causing pronounced intoxication, fever, cough, muscle and head pain. The danger of influenza lies not in its prevalence — which is already enormous — but in the speed of the virus's reproduction and its ability to cause severe complications: pneumonia, myocarditis, exacerbation of chronic diseases, even sepsis. Children under 5, people over 65, pregnant women, and patients with immunodeficiency or chronic diseases of the lungs, heart, kidneys, and diabetes are particularly vulnerable. The flu vaccine is one of the most effective ways to protect against it, but many are still afraid to get it because they have heard: "I got vaccinated — and a week later I got the flu." Is this possible? Yes, but not in the way it seems. Let's sort it out step by step — without fear, without myths, only facts and practical recommendations.

Classification of the disease according to ICD-11

In the International Classification of Diseases 11th Revision (ICD-11), influenza is coded as BA40 — "Influenza caused by influenza virus A" and BA41 — "Influenza caused by influenza virus B." There are also subcodes to specify the form:

• BA40.0 — Influenza A, unspecified;
• BA40.1 — Influenza A(H1N1)pdm09;
• BA40.2 — Influenza A(H3N2);
• BA40.3 — Influenza A(H5N1), H7N9, and other zoonotic strains;
• BA41.0 — Influenza B, unspecified;
• BA41.1 — Influenza B, lineage Victoria;
• BA41.2 — Influenza B, lineage Yamagata.

Important: if influenza is complicated by pneumonia, the code is added CA10 (“Pneumonia caused by the influenza virus”). In the case of secondary bacterial pneumonia, the code for the bacterial pathogen is also added (for example, CA11.0 for pneumococcus). These codes are used in medical records, when issuing sick leaves, and in disease statistics — and they help track which strains are circulating in the current season.

History of the disease and interesting historical facts

Influenza has been known to humanity for many centuries. The first reliable description of an epidemic resembling influenza dates back to 1580 in Italy and Spain — it spread from the Mediterranean to Asia in just a few weeks. But the real breakthrough in understanding came in the 20th century.
The most devastating pandemic — the “Spanish flu” of 1918–1919 — claimed the lives of about 50 million people worldwide. The virus was particularly dangerous for young people aged 20–40 — a group that is usually resistant to respiratory infections. Studies from 2005 showed that the H1N1 virus of that time had a unique hemagglutinin structure that caused a “cytokine storm” — an excessive immune response leading to pulmonary edema and death.
An interesting fact: the name “Spanish flu” is not because the virus originated in Spain. Spain was a neutral country during World War I and did not impose censorship on news about the disease, so it was the first to report the scale of the epidemic. In other countries, information was concealed — to avoid undermining troop morale.
Another important point: in 1933, scientists first isolated the influenza A virus — this marked the beginning of the era of vaccination prevention. The first vaccine appeared in 1945 and was used in the U.S. military. By the 1950s, mass vaccination campaigns began in the civilian sector.

Epidemiology: statistics on the occurrence of the disease

According to WHO, annually influenza affects between 51 million to 101 million adults worldwide and up to 20 million children. This amounts to 1 billion cases of ARVI per year, of which 3–5 million are severe forms requiring hospitalization. Annually, influenza causes the death of 290,000 to 650,000 people — mainly due to complications.
In Russia, the influenza season usually begins in November–December, peaks in January–February, and declines by April. According to Rospotrebnadzor for the 2023/2024 season:

• The incidence rate was 124.3 cases per 10,000 population;
• The highest levels were in the Republic of Tuva (247.1), Khabarovsk Krai (218.4), and Yamal-Nenets Autonomous Okrug (201.9);
• The proportion of hospitalizations was 1.81% of all registered cases;
• The mortality rate from influenza and its complications was 0.0121% (12 per 100,000).

Important: these figures are only laboratory-confirmed cases. The actual number of infected individuals is higher — as many carry influenza “on their feet” without consulting a doctor. It is also worth noting that in recent years there has been an increase in cases of influenza B — especially in seasons with low activity of strain A.

Genetic predisposition to influenza

There is no direct “influenza gene” — but there are genes that influence susceptibility to the virus and the severity of the disease. Research over the past 10 years has identified several key regions:

• Gene IFITM3 (interferon-induced transmembrane protein 3) - regulates the entry of the virus into the cell. In people with the rs12252-C mutation, the risk of severe influenza increases by 6 times. This mutation is more common in Asians (up to 25%) and less common in Europeans (up to 4%).
• Gene CCR5 - a receptor used by some viruses for penetration. The defective form CCR5-Δ32 reduces the risk of HIV but may worsen influenza - the data is contradictory, but a 2021 study showed that carriers of Δ32 have a higher risk of pneumonia with influenza A(H1N1).
• HLA-class II genes (for example, DRB1*07:01) are associated with a weaker immune response to the vaccine. People with these alleles may produce fewer antibodies after vaccination.

This does not mean that "you are doomed." Genetics is just one factor. A healthy lifestyle, timely vaccination, and hygiene minimize the risk even in genetically vulnerable individuals.

Risk factors for flu occurrence

Risk factors can be divided into three groups: biological, behavioral, environmental.
Biological:

• Age: children under 2 years and people over 65 years;
• Chronic diseases: bronchial asthma, COPD, coronary heart disease, heart failure, diabetes, chronic kidney failure;
• Immunodeficiencies: HIV, oncological hematological diseases, use of immunosuppressants;
• Pregnancy (especially II–III trimester) - due to physiological reduction of cellular immunity.

Behavioral:

• Refusal of vaccination;
• Smoking (destroys the ciliated epithelium of the respiratory tract);
• Lack of sleep and chronic stress - reduce interferon production;
• Poor hand hygiene and lack of mask-wearing during the epidemic season.

Environmental:

• Dense groups: schools, kindergartens, offices, public transport;
• Low humidity (<30%) - the virus survives longer in aerosol;
• Air pollution (PM2.5, NO₂) - damages the mucosa and reduces local immunity.

If you belong to one or more risk groups - vaccination is not just recommended, it is vital.

Diagnosis of influenza: how to distinguish it from ARVI and what to check

The main symptoms of influenza - sudden onset, temperature ≥38°C, chills, headache, muscle aches, dry cough, weakness. Unlike ARVI, runny nose and sore throat in influenza are often less pronounced in the first days.
Diagnosis includes three stages:
1. Clinical examination and history:

• Clarification of the date of onset of symptoms;
• Contact with sick individuals;
• Vaccination status in the current season;
• Chronic diseases.

2. Laboratory methods:

3. Differential diagnosis:

• ARVI (adenovirus, rhinovirus) — gradual onset, runny nose and sore throat dominate, temperature below 38.5°C;
• Coronavirus infection (SARS-CoV-2) — loss of smell/taste, shortness of breath, prolonged cough; PCR test resolves the issue;
• Bacterial pneumonia — high temperature >5 days, purulent sputum, localized wheezing on auscultation;
• Meningitis — rigidity of the neck muscles, photophobia, vomiting without nausea.

If symptoms appeared within 48 hours — it is important not to waste time: antiviral drugs work only in the early phase.

Treatment of influenza: when and how to treat

Influenza treatment is not about "curing the virus," but helping the body cope with it and preventing complications. It is divided into three directions:
1. Specific antiviral therapy:

• Oseltamivir (Tamiflu) — neuraminidase inhibitor. Take within 48 hours after the onset of symptoms, course — 5 days. Dose: adults 75 mg twice a day. For children — by weight. Effective against A and B.
• Zanamivir (Relenza) — also a neuraminidase inhibitor, but in the form of inhalations. Not recommended for bronchial asthma.
• Baloxavir marboxil (Xofluza) — a new drug that blocks the cap-dependent endonuclease of the virus. Single dose, but expensive and not yet included in the standards of the Ministry of Health of the Russian Federation.

2. Symptomatic therapy:

• Antipyretics: paracetamol (no more than 4 g/day) or ibuprofen. Aspirin is prohibited in children due to the risk of Reye's syndrome.
• Hydration: at least 2–2.5 liters of fluid per day (water, compotes, rosehip decoctions). Dehydration exacerbates intoxication.
• Rest: at least 5–7 days without physical exertion, even if the temperature has dropped.

3. Prevention of complications:

• Do not prescribe antibiotics without confirming a bacterial infection — they do not act on viruses and can cause dysbiosis.
• For cough — expectorants (ambroxol, acetylcysteine), but not antitussives (codeine) — they delay sputum.
• In case of severe intoxication — intravenous solutions (glucose, reosorbilact) in a hospital.

Important: treatment should be prescribed by a doctor. Self-medication — especially the combination of antipyretics and antivirals without supervision — is dangerous.

List of medications used for influenza

Here is the current list of medications approved for use in the Russian Federation as of 2026, indicating the form of release and features:

Note: Arbidol is included in the national recommendations of the Ministry of Health of the Russian Federation, but is not recognized as effective in international meta-analyses. If you are choosing between Tamiflu and Arbidol — it is better to take the former, especially if there are risk factors.

Disease monitoring: control, prognosis, and complications

After diagnosis, it is important to monitor dynamics. Control stages:

• Day 1–2: temperature, respiratory rate, blood oxygen saturation (SpO₂). Normal SpO₂ ≥95%. If below 94% — hospitalization is needed.
• Day 3–4: assessment of therapy effectiveness: reduction in temperature, decrease in intoxication. If the temperature does not drop — a bacterial superinfection is possible.
• Day 5–: disappearance of fever, restoration of appetite and strength. If cough worsens, purulent sputum appears — a chest X-ray is performed.

Forecast:

• In healthy individuals — complete recovery in 7–10 days;
• In individuals with risk factors — complications may occur in 5–10% of cases;
• Mortality with timely treatment is less than 0.1%.

Complications (in order of frequency):**

Influenza in children, adults, and the elderly is almost different diseases.
Children under 3 years:

• Often there is no classic fever — there may be subfebrile temperature (37.5–38°C) or even normal temperature in severe condition;
• Symptoms of intoxication dominate: lethargy, refusal to eat, vomiting;
• High risk of seizures with fever;
• Laryngotracheitis ("false croup") often develops — hoarseness, "barking" cough, stridor.

Adolescents and young adults (15–40 years):**

Elderly (≥65 years):**

Question 1: Can you get infected with influenza after vaccination? And if so — why?
Yes, you can — but not from the vaccine itself. Modern influenza vaccines contain **inactivated** (killed) viruses or their fragments — they cannot cause the disease. However, three scenarios are possible:

• Incubation period: you were already infected before vaccination, but symptoms appeared 1–2 days after it;
• Strain mismatch: the vaccine contains 3–4 strains, but another one is circulating (e.g., drifted strain). Effectiveness in this case drops to 30–40%, but still reduces severity;
• Weak immune response: in elderly or immunocompromised individuals, antibody production may be insufficient.

The vaccine does not provide 100% protection, but reduces the risk of hospitalization by 40–60% and death by 70–80%.
Question 2: How many days after vaccination does immunity form?
Full immune response develops within **14–21 days** after vaccination. Therefore, vaccination should be done before the start of the epidemic season — ideally in September–October. If done in December — protection will be partial, but still better than nothing.
Question 3: Why does the arm hurt and the temperature rise after vaccination?
This is a normal reaction to the introduction of a foreign protein. At the injection site — local inflammatory reaction (pain, redness, swelling), in the body — short-term fever (up to 38.5°C), weakness. Lasts no more than 48 hours. This is not influenza, but a sign that the immune system is "working." If the temperature is above 38.5°C or lasts more than 2 days — consult a doctor.
Question 4: Can you get vaccinated if you are already sick with ARVI?
Yes, but only with a mild course without fever. If you have a runny nose and cough, but the temperature is 36.8°C — vaccination is permissible. If the temperature is ≥38°C — postpone for 1–2 weeks after recovery. Vaccination during acute infection may result in a weak immune response.
Question 5: Do you need to get vaccinated every year?
Yes. The influenza virus mutates quickly — new strains circulate every season. The vaccine is updated annually based on WHO recommendations. Even if you had influenza last year — immunity to new strains does not guarantee protection. Annual vaccination is the only reliable way to prepare the immune system for the upcoming season.

Typical mistakes with influenza and how to avoid them

• Error: "I'll take aspirin - it will reduce the fever faster."
  Consequences: in children - Reye's syndrome (acute liver failure + encephalopathy), in adults - gastrointestinal bleeding.
  What to do: use only paracetamol or ibuprofen. Aspirin - only as prescribed by a doctor for CAD.
• Error: "I'll take antibiotics 'just in case'."
  Consequences: dysbiosis, bacterial resistance, sputum retention, worsening condition.
  What to do: antibiotics are prescribed by a doctor only upon confirmation of bacterial infection (based on blood tests, X-rays, sputum).
• Error: "The fever has gone down - I can go to work."
  Consequences: relapse, myocarditis, chronicization of the process.
  What to do: rest for at least 5-7 days after the fever subsides. Physical activity - not earlier than in 2 weeks.
• Error: "I got vaccinated - now I'm invincible."
  Consequences: neglecting hygiene, infection and transmission of the virus to others.
  What to do: vaccination is a shield, not armor. Continue to wash your hands, wear a mask in transport, avoid crowds during the epidemic season.

Conclusion: what is important to know about influenza and vaccination

Influenza is not a "cold," but a serious infectious disease with potentially fatal consequences. The flu vaccine is the most accessible and proven way to reduce severe cases. Yes, sometimes reactions occur after it - but this is not an illness, but a sign of the immune system working. Yes, you can get the flu after vaccination - but 3-5 times less often, and 5-7 times milder.
If you are at risk - get vaccinated annually, ideally by October. If you get sick - don't wait, start antiviral therapy within the first 48 hours. Do not self-medicate, especially with antibiotics. And remember: your task is not just to survive the flu, but to survive it without complications. For this, three things are enough: vaccination, timely consultation with a doctor, and respect for your body.
I am Dr. Korzhikov, and I want you not to fear the flu, but to know how to cope with it. Because knowledge is not only prevention, it is confidence in every day.