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

Abstract

Population immunity (herd 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 (N=501) who had never been positively diagnosed with or vaccinated against SARS-CoV-2. Serum samples were collected in Wrocław (Lower Silesia) on 15th and 22nd May 2021. Sera from hospitalized COVID-19 patients (N=22) or from vaccinated citizens (N=14) served as positive controls.Sera 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, and antigens for exclusion cross-reactivity with other coronaviruses: MERS-CoV, SARS-CoV, HCoV 229E Np, HCoV NL63 Np.Within the investigated population of healthy individuals 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 52.3% (262 out of 501) were positive for SARS-CoV-2-specific IgA; 21.2% 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 vaccination against COVID-19 is still of the most importance for controlling the pandemic.

Dynamics of anti-SARS-CoV-2 seroconversion in individual patients and at the population level

“…The same individuals (thus also 26%) demonstrated positive levels of anti-RBD IgG induced by the infection. Interestingly, the ‘dark figure’ from the previous pandemic waves, which was assessed in patients representing the same geographical region in May 2021, was highly comparable, at 25.6% [7]. Since pandemic waves before May 2021 in Poland were caused by the Alpha variant of the virus, while between May and December 2021 the Delta variant dominated, our observation indicates that the ‘dark figure’ has not changed with the SARS-CoV-2 variant.…”

“…In this study we continued population screening for hidden herd immunity to COVID-19 initiated in May 2021 [7], and we found that specific antibodies targeting SARS-CoV-2 that developed before May 2021 cannot be considered important 7 months later, in December 2021. The only factor that provided a good level of population immunity to COVID-19 was vaccination.…”

“…This study is a follow-up of the anti-SARS-CoV-2 antibody screening completed on 15 and 22 May 2021 in healthy individuals who had not been previously vaccinated against COVID-19 and had never been positively diagnosed for SARS-CoV-2 infection [7]. In the period between May and December 2021 the fourth wave of COVID-19 occurred, with the Delta variant contribution assessed as almost 100% from mid-October until the end of the year.…”

“…Despite over 440 million confirmed cases of COVID-19, the true prevalence of infection remains significantly underestimated due to a fraction of asymptomatic and oligosymptomatic infections as well as to limited abilities of robust diagnostics worldwide [4,5,6,7]. The serological response and the association with clinical symptoms are important in understanding the pathogenesis of COVID-19 [8].…”

“…Here we present a follow-up study for a population screening for SARS-CoV-2 specific antibodies in Polish citizens (N=501) who had never been positively diagnosed with or vaccinated against SARS-CoV-2 [7]. In that group we identified a significant, hidden fraction of individuals who had developed anti-SARS-CoV-2 antibodies.…”