Hidden fraction of Polish population immune to SARS-CoV-2 in May 2021

Abstract

Population immunity to SARS-CoV-2 derives from two well-defined and controlled sources: vaccinations or diagnosed and registered cases of the disease. It may however also result from asymptomatic, oligosymptomatic, or even full-blown but undiagnosed and unregistered cases from which patients recovered at home. Here we present a population screening for SARS-CoV-2 specific IgG and IgA antibodies in Polish citizens (healthy adults, N=501) who had never been positively diagnosed with or vaccinated against SARS-CoV-2. Blood samples were collected in Wrocaw (Lower Silesia) on 15th and 22nd May 2021. Sera from COVID-19 patients with a severe course (hospitalized) (N=43) or had been vaccinated (N=14) served as a positive control. The patients were tested with Microblot-Array COVID-19 IgG and IgA (quantitative) that contain specific SARS-CoV-2 antigens: NCP, RBD, Spike S2, E, ACE2, PLPro protein, as well as antigens for exclusion cross-reactivity with other coronaviruses: MERS-CoV, SARS-CoV, HCoV 229E Np, HCoV NL63 Np. Within the investigated population of healthy adults who had never been positively diagnosed with or vaccinated against SARS-CoV-2, we found that 35.5% (178 out of 501) were positive for SARS-CoV-2-specific IgG and 52.5% (263 out of 501) were positive for SARS-CoV-2-specific IgA; 21.6% of the investigated population developed virus-specific IgG or IgA while being asymptomatic. Anti-RBD IgG, which represents virus-neutralizing potential, was found in 25.6% of individuals (128 out of 501). These patients, though positive for anti-SARS-CoV-2 antibodies, cannot be identified in the public health system as convalescents due to undiagnosed infections, and they are considered unaffected by SARS-CoV-2. Their contribution to population immunity against COVID-19 should however be considered in predictions and modeling of the COVID-19 pandemic. Of note, the majority of the investigated population still lacked anti-RBD IgG protection (74.4%); thus the positive fraction is not sufficient for effective population immunity, and vaccination against COVID-19 is still of the most importance for controlling the pandemic.