Antibodies to influenza: how they work and why they are not enough

Influenza is not just a "cold with a fever." It is an acute viral respiratory disease caused by RNA viruses of the genus *Influenzavirus*, which can paralyze the operations of entire enterprises in just a few days, overload hospitals, and, in severe cases, lead to death. Unlike ARVI, influenza develops rapidly: within 1-2 days after infection, a person may feel as if they have been "hit by a truck" — joint pain, fever up to 40 °C, headache, weakness, cough, nasal congestion. And although most cases end with recovery within 7-10 days, influenza remains one of the main causes of seasonal hospitalizations and sudden death among the elderly and people with chronic diseases. It is particularly concerning that even after the infection, immunity to the virus is incomplete — and this is where antibodies come into play: powerful, but not omnipotent defenders of our body.

Classification of the disease according to ICD-11

According to the International Classification of Diseases 11th Revision (ICD-11), influenza falls under the section "Infectious and parasitic diseases" and is coded as **BA43** — "Influenza caused by influenza virus A or B." Subcategories detail the type of virus and clinical form:

• BA43.0 — Influenza caused by influenza virus A (for example, subtype H1N1, H3N2)
• BA43.1 — Influenza caused by influenza virus B
• BA43.2 — Influenza caused by influenza virus C (rarely causes severe forms)
• BA43.Y — Other specified forms of influenza
• BA43.Z — Unspecified influenza

It is important to understand: coding not only helps doctors in statistical management but also directly affects the allocation of funds for prevention and treatment. For example, in Spain, more than 95 % cases of influenza are registered annually as BA43.0 or BA43.1 — these strains are included in the seasonal vaccine composition. However, influenza C (BA43.2) almost never causes epidemics and therefore is not included in the vaccination strategy of the European Centre for Disease Prevention and Control (ECDC).

The history of influenza: from the "Spanish flu" to the pandemics of the 21st century

Influenza has been known to humanity for a long time — the first reliable descriptions date back to the 15th century, but a real breakthrough in understanding the disease occurred only in 1933 when British scientists Wilson Smith, Christopher Andrews, and Patrick Laidlaw first isolated the influenza A virus from the nasal mucus of an infected patient. Before that, influenza was considered a bacterial infection.
The most destructive pandemic in history — the "Spanish flu" of 1918–1919 — claimed about 50 million lives worldwide. The name "Spanish" is a historical mistake: in Spain, a neutral country during World War I, there was no censorship, so outbreaks were reported freely. In other countries, information was concealed — and the myth of "Spanish" origin arose. In fact, the first cases were recorded in the American military camp Fort Riley (Kansas) in March 1918.
In the 21st century, we faced two significant pandemics:

• 2009 year — the H1N1 pandemic ("swine flu"). The virus contained fragments of swine, avian, and human influenza. In Spain, over 1.2 million cases were recorded during the season, with 300 deaths.
• 2020–2022 — the COVID-19 pandemic temporarily "displaced" influenza: thanks to masks, quarantines, and social distancing, the circulation of influenza viruses sharply declined. But in the 2022/2023 season, influenza returned with renewed strength — especially in Europe, where there is an increase in morbidity among children and young adults.

An interesting fact: the influenza A virus has a unique ability for "reassortment" — when two different strains simultaneously infect one cell and exchange genetic material. This is how the pandemic strains of 1957 (H2N2), 1968 (H3N2), and 2009 (H1N1pdm09) arose. This makes influenza one of the most variable viruses on the planet.

The epidemiology of influenza worldwide: numbers, seasons, and trends

According to WHO, annually, influenza affects 5 % to 10 % of the adult population worldwide and 20 % to 30 % of children. On a global scale, this amounts to 1 billion cases, 3–5 million severe forms, and 290–650 thousand deaths per year. Countries with a temperate climate suffer the most — there are clearly defined seasonal outbreaks: in the Northern Hemisphere — from October to March, in the Southern — from April to September.
In Spain, the situation is typical for Western Europe:

• The influenza season usually begins in December, peaking in January–February.
• In the 2023/2024 season, according to the Instituto de Salud Carlos III, 142,000 laboratory-confirmed cases were recorded, including 12,500 hospitalizations and 1,840 deaths (mostly among individuals over 65 years old).
• Children under 5 years old have the highest morbidity rate — up to 250 cases per 10,000 population, but low mortality. Among the elderly (over 85 years old), it is the opposite: morbidity is lower (about 100 per 10,000), but lethality reaches 15 %.

There is particular concern about the increase in cases among people with obesity, diabetes, and COPD — in the last 5 years, their share among hospitalized patients has increased by 22 %. This is related not only to weakened immunity but also to impaired lung function and systemic inflammatory response.

Genetic predisposition to severe influenza

Genetics does not determine whether "you will get infected," but it significantly affects how your body will respond to the virus. Recent studies have identified several key genes associated with an increased risk of severe influenza:

• IFITM3 — a gene that encodes a protein that blocks the virus from entering the cell. The mutation rs12252-C increases the risk of hospitalization by 6 times. This variation is more common in Asians (up to 25 %), less so in Europeans (4–6 %).
• TLR7 — a receptor that recognizes viral RNA. Women with mutations in this gene (X-linked form) may have a reduced interferon response — explaining why some women experience more severe influenza despite having a stronger baseline immune response.
• HLA-DRB1*07:01 — a variant of the major histocompatibility complex gene associated with poor recognition of influenza virus epitopes. People with this allele are less likely to form long-term T-cell responses.

Important: the presence of such genes does not mean that you will necessarily become seriously ill. But if you have a family history of severe forms of influenza (for example, first-degree relatives had hospitalizations or fatalities), it is worth discussing preventive measures with your doctor — including early vaccination and antiviral therapy at the first symptoms.

Risk factors: what makes a person vulnerable to influenza

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

• Age over 65 or under 2 years
• Chronic diseases: COPD, asthma, heart failure, diabetes, immunosuppression (HIV, oncology, use of glucocorticoids)
• Pregnancy (especially the II–III trimester) — due to physiological immunosuppression and increased load on the lungs

Behavioral:

• Smoking — reduces mucociliary clearance and damages the respiratory epithelium
• Lack of sleep and chronic stress — suppress interferon production
• Refusal of vaccination — the most significant modifiable risk factor

Environmental:

• Dense groups: schools, offices, nursing homes
• Low humidity (less than 40 %) — the virus remains viable longer, mucous membranes dry out
• Air pollution (PM2.5, NO₂) — exacerbates inflammation in the lungs and reduces barrier function

If your task is to minimize risk — start not with "antiviral teas," but with controlling these factors. For example, in Spanish offices with air conditioning, it is recommended to maintain humidity at 45–55 % and ventilate every 2 hours — this reduces virus transmission by 30 % according to a study by Universidad de Barcelona (2022).

Diagnosis of influenza: how to distinguish it from ARVI and when laboratory tests are needed

Clinically, influenza is often confused with other ARVI, but there are "red flags" indicating it:

• Acute onset (within 1–2 hours)
• High fever (>38.5 °C) lasting 3–5 days
• Muscle aches, headache, weakness, "aches all over"
• Cough — dry, paroxysmal, appears after fever
• Absence of a runny nose in the first 2 days (unlike ARVI)

The following methods are used to confirm the diagnosis:

Differential diagnosis includes:

• COVID-19 — similar symptoms, but more often — loss of smell, diarrhea, prolonged cough
• RSV — especially in children: bronchiolitis, difficulty breathing
• Adenovirus — conjunctivitis + fever + pharyngitis
• Bacterial pneumonia — worsening of symptoms after day 5–7, purulent sputum

If you notice a sharp deterioration in yourself or a close person after 3-4 days of fever, this is a signal for immediate consultation: secondary bacterial pneumonia may be possible.

Treatment of influenza: when medications help and when only rest is needed

Treating influenza is not about fighting the virus head-on, but about supporting the immune system and preventing complications. The main principles are:
1. Antiviral therapy (only with early prescription)
Neuraminidase inhibitor drugs (oseltamivir, zanamivir) are effective if started within the first 48 hours of symptom onset. They reduce the duration of fever by 1-1.5 days and the risk of hospitalization by 60%. In Spain, oseltamivir is available by prescription and is recommended for:

• People over 65 years old
• Pregnant women
• Patients with chronic diseases
• Anyone hospitalized with suspected influenza

2. Symptomatic therapy
Here it is important not to overdo it:

• Antipyretics: paracetamol (no more than 3 g/day) is safer for the liver than ibuprofen in hyperthermia.
• Cough suppressants: codeine or butamirate — only for dry, painful cough without phlegm. Do not give to children under years.
• Moisturizing mucous membranes: saline sprays (saline solution), inhalations with warm steam — help reduce viral adhesion.

3. Folk methods — what works and what doesn’t
— Works: plenty of fluids (water, unsweetened compotes), rest, humid air.
— Does not work: high doses of vitamin C (does not reduce the incidence of influenza), garlic in the nose (injures the mucosa), hot baths during fever (worsen dehydration).
Important: antibiotics for influenza **are not needed** if there are no signs of bacterial infection. Their prescription increases the risk of dysbiosis and resistance without benefit.

List of medications used for influenza in Spain

Here is the current list of drugs registered in Spain and recommended by the Ministry of Health (Ministerio de Sanidad) and ECDC for 2025:

Note: Xofluza® is available in Spain by prescription and costs about 85 euros for a course. Its advantage is a single dose, but it is less studied in pregnant women and children under 12 years old.

Disease monitoring: when to be concerned and which complications are dangerous

After the start of treatment, it is important to monitor the dynamics. Control stages:

• Day 1–2: fever should decrease by 0.5–1 °C/day. If the temperature remains >39 °C — resistance or bacterial superinfection is possible.
• Day 3–4: cough should become productive (with phlegm), and the overall condition should improve. If weakness increases — check for pneumonia.
• Day 5–7: fever should be below 37.5 °C. If it rises again — this is a "second wave": often bacterial pneumonia or myocarditis.

The most dangerous complications:

• Secondary bacterial pneumonia — most often caused by Streptococcus pneumoniae or Staphylococcus aureus. Symptoms: worsening cough, purulent sputum, shortness of breath, localized chest pain.
• Myocarditis and pericarditis — especially in young people. Complaints: pain behind the sternum, arrhythmia, low blood pressure.
• Encephalopathy/encephalitis — rare, but possible in children (Reye's syndrome with aspirin use).
• Poliomyelitis-like syndrome — muscle weakness after influenza, requires MRI and EMG.

The prognosis is favorable in 95% of healthy people. But in patients with severe comorbidities, mortality can reach 10–15%. Therefore, it is important not to ignore "mild influenza" — especially if you are in a risk group.

Age-related features: how influenza "adapts" to age

Influenza is not a universal disease. Its manifestation depends on age and the state of the immune system.
Children under 2 years:
Often there is no classic fever — instead, there is lethargy, refusal to eat, vomiting, diarrhea. High risk of bronchiolitis and obstructive syndrome. Antibodies to influenza develop more slowly in them, so recurrent infections in the first year of life are normal. In Spain, vaccination is recommended from 6 months (2 doses with an interval of 4 weeks).
Children 2–12 years:
The highest incidence. Often — high fever, seizures (at temperatures >39 °C), otitis. The immune system is still "learning" — therefore, antibodies to the new strain are formed, but not always persistently (long-term memory is weaker than in adults).
Adolescents and young adults (15–40 years):
Most often mild course, but in pandemic seasons (like in 2009) — high risk of severe forms due to "cytokine storm": the immune system overreacts. This group most often ends up in intensive care with H1N1.
Elderly (over 65 years old):
Fever may be moderate (37.5–38.5 °C), but respiratory failure develops quickly. Antibodies to the flu are produced less effectively due to immune aging (inflammaging). In Spain, adjuvant vaccines (e.g., Fluad® with MF59) are recommended for this group — they increase the response by 50–70% compared to standard ones.

Questions and answers: the most frequent patient inquiries

Question 1: Why did I get vaccinated against the flu but still got sick?
This is not a vaccine error — it is a feature of the virus. The vaccine contains 3–4 strains predicted by WHO for the season. But if the virus has mutated (antigenic drift), the antibodies will be less effective. However, even in this case, the vaccine reduces the severity of the disease: vaccinated individuals have a 40–60% lower risk of hospitalization and a 70% lower mortality rate. It's like a "bulletproof vest": it doesn't guarantee you won't get hit, but it will save you from a fatal injury.
Question 2: Can I get vaccinated if I already have a runny nose but no fever?
Yes, if it is a mild ARVI without fever — vaccination is not contraindicated. Contraindications: fever >38 °C, exacerbation of a chronic disease, anaphylaxis to egg white (although modern vaccines contain it in microdoses). In Spain, vaccination is even performed on the day of the visit — if there are no contraindications.
Question 3: How long does the flu virus live on surfaces?
On metal and plastic — up to 24–48 hours, on paper and fabric — up to 8–12 hours. But the infectious dose is small: only 100–1000 particles are enough. Therefore, regular disinfection (70% alcohol, chlorine-containing agents) and hand washing are the simplest but effective measures. In Spanish schools, it is now mandatory to disinfect desks every 2 hours during an epidemic.
Question 4: Does vitamin D help with the flu?
Yes, but not as a treatment, rather as prevention. Vitamin D deficiency (less than 20 ng/ml) is associated with an increased risk of ARVI. In the winter months, the vitamin D level in 40% of adults in Spain is below normal. The recommended preventive dose is 800–2000 IU/day. However, adding vitamin D once the flu has started will not speed up recovery.
Question 5: Why do I have a cough and weakness for a month after the flu?
This is post-viral asthenia and reflex cough. After the flu, the bronchial mucosa remains hyperreactive for 3–6 weeks. Coughing is a protective reaction to clear the airways. If the cough is dry, without phlegm, and not accompanied by shortness of breath — this is normal. But if there is blood in the phlegm, you have lost more than 5% of your weight, or the cough lasts longer than 6 weeks — a consultation with a pulmonologist is needed.

Typical mistakes in treating influenza and how to avoid them

• Mistake 1: "I will take an antibiotic 'just in case'"
  → Consequences: dysbiosis, resistance, allergy.
  → How to avoid: an antibiotic is prescribed only by a doctor upon confirmation of a bacterial infection (by blood test, X-ray, sputum).
• Mistake 2: "I will wait out the flu at home — it will go away on its own"
  → Consequences: late hospitalization, pneumonia, complications.
  → How to avoid: if you are over 65, pregnant, or have chronic diseases — contact a medical center within the first 48 hours. In Spain, many clinics offer "rapid flu screening" in 30 minutes.
• Mistake 3: "I will do 5 inhalations with essential oils — and everything will pass"
  → Consequences: mucosal burns, bronchospasm, worsening breathing.
  → How to avoid: use only saline solution or alkaline inhalations (sodium bicarbonate 2%). Essential oils — only in a diffuser, not in a nebulizer.
• Error 4: "I don't get vaccinated — I have a strong immune system"
  → Consequences: you may become a source of infection for the elderly and children.
  → How to avoid: vaccination is not just about you. In Spain, 70% of flu deaths are due to contact with unvaccinated people.

Conclusion: antibodies are not a panacea, but an important link in protection

Antibodies to the flu are a powerful but not perfect mechanism of protection. They work like "locks" that recognize the virus by its "key" (hemagglutinin), but if the virus changes the shape of the key — the lock won't work. That’s why vaccination every year is not a sign of a weak immune system, but a manifestation of common sense. In combination with hygiene, risk factor control, and timely treatment, it provides maximum protection.
If you want to minimize the risk:

• Get vaccinated annually — preferably in October-November
• At the first symptoms — call a doctor, don’t wait for it to "pass on its own"
• Maintain humidity, wash your hands, avoid crowds during the epidemic season
• Don’t forget about vitamin D and sleep — they are the foundation of immunity

The flu cannot be defeated once and for all — but it can be made manageable. And remember: even if you get sick — it’s not a failure, but part of the biological process. The main thing is to prevent it from progressing to a severe form. I, Dr. Korzhikov, have been working in Madrid for 12 years — and every season I see how simple measures save lives. Be attentive to yourself and your loved ones.