The aim of the study is to evaluate simultaneously the immunological and clinical efficacy and tolerability of an influenza vaccine, inactivated, quadrivalent, with cleaved virus, in patients at risk for severe and complicated influenza routinely vaccinated against influenza in family medicine clinics or specialty clinics (pediatric, internal medicine, cardiology, gynecological diabetes, pregnant women, transplant).

Influenza has been called the last uncontrollable scourge of mankind because of its epidemic and pandemic potential. According to WHO estimates, 5-10% of the adult population and 10-20% of children globally fall ill with influenza seasonally. Influenza incidence has implications for individual patients (risk of complications, hospitalization, disability and death), as well as for public health (increased outpatient consultations, hospitalizations, including in intensive care units, increased absenteeism from work). Groups at risk for post-influenza complications include children under the age of 5, seniors over 65, patients with chronic diseases such as respiratory, cardiovascular, metabolic conditions, obesity, immunosuppressive treatment, and pregnant women, among others. Since the influenza A H1N1pdm09 pandemic, the WHO has particularly emphasized the need to vaccinate patients at risk of severe and complicated influenza, and has recommended a tetravalent vaccine since 2013. The inactivated tetravalent influenza vaccine has been available in Poland since the 2016/17 season, and also registered for children since the 2018/19 season. This is also in line with current Polish recommendations issued by the National Influenza Control Program. With the extremely low level of vaccination against influenza of the Polish population (about 3.7%), the threat of an influenza epidemic is serious. The number of reports describing simultaneously the immunological and clinical efficacy and tolerance of influenza vaccination of the Polish population is limited, the published works usually focus on the evaluation of either immunogenicity or clinical efficacy, there is a lack of publications comprehensively determining the effectiveness of the vaccine in the Polish homogeneous population, especially people in the above-mentioned risk groups, as well as the determination of the occurrence of the phenomenon of non-immunogenicity of the vaccine in the Polish population. An important aspect in the situation of low immunogenicity of the Polish population is the aspect of persistence of protective antibody levels at the beginning of the next flu season. Few works indicate the persistence of protective antibody levels in healthy individuals without risk factors for immune disorders, but the quality of "immune memory" after influenza vaccination in patients at risk of severe and complicated influenza is still insufficiently understood.

The aim of the study is to evaluate simultaneously the immunological and clinical efficacy and tolerability of an influenza vaccine, inactivated, quadrivalent, with cleaved virus, in patients at risk for severe and complicated influenza routinely vaccinated against influenza in family medicine clinics or specialty clinics (pediatric, internal medicine, cardiology, gynecological diabetes, pregnant women, transplant).

Inclusion criteria for the study:

1. age over 18 (adults able to give informed consent to participate in the study) or age under 18 and the consent of a legal guardian and, in the case of persons aged 16-18, also the consent of a minor;
2. belonging to groups at risk of severe and complicated course of influenza (children, elderly people, patients with chronic diseases, pregnant women);
3. no clinical contraindications to influenza vaccination;
4. the patient and/or his legal guardians give written, informed consent to participate in the study.

Exclusion criteria from the study:

1. lack of consent to participate in the study - at every stage of the study;
2. clinical contraindications to influenza vaccination.

Clinical contraindications to influenza vaccination:

• exacerbation of a chronic disease;
• acute infectious disease;
• allergy to a vaccine component;
• a serious allergic reaction after a previous flu vaccination;
• Guillain-Barre syndrome up to 6 weeks after the previous vaccination;
• use of immunosuppressive treatment (relative contraindication)

In the arm concerning pregnant patients, only women