Lack of knowledge around seroprevalence levels of COVID-19 in Poland was the reason for the implementation of a seroepidemiological study in the Katowice Region (2,100,000 inhabitants). In October–November 2020, a questionnaire examination and measurement of anti-SARS-CoV-2 IgG and IgM antibodies were performed in a random sample of the general population (n = 1167). The objectives of the study were to estimate the prevalence of IgG and IgM antibodies and to assess their host-related correlates. The prevalence of IgG seropositivity was 11.4% (95% CI: 9.5–13.2%) and IgM seropositivity was 4.6% (95% CI: 3.5–5.8%). Diagnosis of COVID-19 was found in 4.8% of subjects. A positive IgG test was statistically significantly associated with age (inverse relationship), a person’s contact with a COVID-19 patient, quarantine, and two symptoms in the past: fever and loss of smell/taste. Positive IgG tests were less prevalent in subjects who had diagnoses of arterial hypertension, diabetes, or rheumatologic disorders. IgM test positivity was associated with quarantine and loss of smell/taste only with no effect of chronic diseases found. In Poland, in the period October–November 2020, the prevalence of SARS-CoV-2 infection was larger than earlier estimates obtained in other European countries, probably reflecting the measurements obtained during the “second wave” of the epidemic.

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Background: According to published data the number of deaths attributed to COVID-19 is underestimated between 30 and 80%. Aim: The aim of this study is to assess the impact of COVID-19 on total mortality of Poland and the Silesian voivodship. Methods: Secondary epidemiological data on COVID-19 deaths were obtained from the Ministry of Health registry and data on total mortality were gathered from the National Statistical Office and Registry Office in Poland. Three scenarios were used to estimated COVID-19 deaths: real number + an extra 30%, 60%, and 70% excess total deaths. Results: In 2020, there were 73,254, 64,584, and 67,677 excess deaths in comparison to 2017–2019, respectively. For the Silesian voivodship, it was 8339, 7946, and 8701, respectively. The total mean increase in deaths was 16% for the whole country and the Silesian voivodship. The simulation for 30% extra COVID-19 deaths gave COVID-19 mortality equal to 12.5%; n = 50,708 deaths, for extra 60%; 17.9% n = 72,866 and for extra 70%; 19.7% n = 80,251 for Poland; and 11.9% (n = 6072), 17.2% (n = 8740), 24.2% (n = 12,297), respectively, for the Silesian voivodship. Conclusions: The participation of COVID-19 in total deaths should not exceed 20% for Poland and 24% for the Silesian voivodship in 2020.

The explanation of the potential interaction between the influenza vaccine and SARS-CoV-2 infection is urgently needed in the public health. The objective of the study is to compare the occurrence of positive SARS-CoV-2 IgG and IgM tests in subjects with and without recent (last year) seasonal influenza vaccinations. In a cross-sectional study located in three large towns of Silesian Voivodeship (Poland), we studied 5479 subjects in which 1253 (22.9%) had a positive anti-SARS-CoV-2 IgG test and 400 (7.3%) had a positive anti-SARS-CoV-2 IgM test. Seasonal influenza vaccination remains an independent factor protecting against positive IgG tests (OR = 0.68; 0.55–0.83). The effect is not apparent with IgM antibodies. The obtained results confirmed that the serological status of SARS-CoV-2 infection depends on vaccination against seasonal influenza.

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