The 2019 coronavirus disease (COVID-19) pandemic is caused by a novel coronavirus named “Severe Acute Respiratory Syndrome Coronavirus 2” (SARS-CoV-2) and has become a major worldwide challenge since December 2019 [1]. The person-to-person transmission of COVID-19 occurs during the pre-symptomatic phase through direct contact or droplets [2-4]. Because asymptomatic cases can spread the infection in the population [5], the probability of transmission in the pre-symptomatic phase of this infection has led to challenges in controlled measures [6, 7]. Although Reverse Transcription of the Polymerase Chain Reaction (RT-PCR) is currently considered the “standard test” for diagnosing SARS-CoV-2 infection [8], studies have shown a significant number of asymptomatic or subclinical individuals remain undetected by RT-PCR. Thus, it is likely that the actual number of people exposed or infected are underestimated [9-11]. Due to the high proportion of asymptomatic and mild infections, it is difficult to estimate the true prevalence of the infection and its mortality rate [12].

Serological tests are important diagnostic methods, especially in patients with mild to moderate infected cases. The use of serological antibody tests, including immunoglobulin G (IgG) and M (IgM) can help estimate the true number of COVID-19 infections [13, 14]. The detection of SARS-CoV-2 specific IgG and IgM in blood serves as a method for determining whether the individual has been infected recently (IgM) or earlier (IgG); IgM usually increases after seven days of infection before increasing IgG. IgG is a longer-lasting antibody which develops 15 to 30 days after the onset of infection [11, 15]. Notably, several studies have reported that approximately 38 to 48% of the population with seropositivity had no symptoms of Covid-19 [16-18]. Therefore, the identification of IgM and IgG antibodies against SARS-CoV-2 can be considered a complementary method for diagnosis and play a significant role in assessing immune responses against SARS-CoV-2 infection. Furthermore, due to the persistence of antibodies to SARS-CoV-2 (mainly IgG) after virus clearance, a serological method provides accurate data to estimate the prevalence of SARS-CoV-2 infection and exposure to it in a population. Given that the seroprevalence of infection reflects the immune status of individuals or the community [19], it can assist policymakers in designing and implementing prevention interventions, quarantine, and predicting medical facilities. Therefore, the aim of this study was to evaluate the seroprevalence of SARS-CoV-2 infection in the general population of Kermanshah province in 2021.

Kermanshah province has experienced three peaks of the COVID-19 epidemic so far, and according to prediction, the fourth peak is also possible (Fig. 1). A total of 1967 people from 14 cities of the province participated in the study during the third peak. 25% of the selected individuals were reluctant to participate and were consequently replaced. The mean age of participants was 35.7±16.9, and 50.4% of participants were female.

Table 1 shows the seroprevalence of SARS-CoV-2-specific antibodies according to the demographic characteristics of the population under study. The seroprevalence of IgG or IgM antibodies in men and women was 184 (18.6) and 214 (21.9), respectively. The highest seroprevalence of IgG or IgM was observed in individuals aged 60 years or older (22.3%) but not much different from other age groups. One hundred twenty-six participants had a history of Covid 19, 50% of whom had IgG or IgM antibodies. Also, 11.9% of them had high IgM antibodies, which is a sign of a recent infection. The seroprevalence of IgG or IgM in the urban population was higher than rural population (22.8 vs. 15.3%) and this difference is statistically significant (p=<0.001). Also, serum prevalence based on a history of close contact, previous infection, and symptoms were 20.8, 4.0, and 29.2 percent, respectively. These differences are statistically significant (p=<0.001)

Fig. (2) shows the sample size and seroprevalence of SARS-CoV-2-specific IgG and IgM antibodies. As can be seen, the lowest IgM, IgG, IgM and IgG, and IgM or IgG seroprevalence was found in Paveh, 0.5, 1.8, 0, and 2.3, respectively. On the other hand, the highest IgM seroprevalence was found in Sahneh (11.9% [7.5–16.3]) and Songhor (5% [5–18.7]) cities. Also, the highest IgG seroprevalence was found in Harsin (59.7% [50.1–69.4]). In addition, the highest seroprevalence of SARS-CoV-2-specific IgG and IgM and IgM or IgG antibodies were related to Songhor (8.3% [1.4–15.2]) and Harsin (61.6% [54.7–68.5]); respectively.

Table 2 demonstrates the Case Fatality Rate (CFR) and Infection Fatality Rate (IFR) in the population under study based on sex, age, and city. As illustrated, the CFR and IFR in men and women were 3.4 vs. 3.3 and 0.1 vs. 0.3, respectively. The highest CFR and IFR were observed in individuals aged 60 years or older, with 11.2 and 3.7%, respectively. In addition, the highest and lowest rates of CFR were found in Dallaho and Salas Bababjani cities, with 6 and 2.1%, respectively. Likewise, the highest and lowest rates of IFR were found in Gilan Gharb and Salas Bababjani cities, with 0.8 and 0%, respectively.

Fig. (1). Time trend of COVID-19 epidemic in Kermanshah province.

Fig. (2). Sample size and seroprevalence percentage of SARS-COV-2-specific IgG and IgM of patients’ antibody in different cities.

The results of this study also showed the self-report for morbidity, history of close contact, close contact with a family member with a seropositive test and a history of the previous infection was 126 (6.4), 200 (10.2), 182 (9.25%), respectively. In addition, 150 (41.3%) and 81 (23.7%) of these subjects had at least one symptom of coronavirus and at least one underlying disease, respectively. In people with a seropositive test or history of the previous infection, 126 (6.4%) were diagnosed by a physician, and 6 (0.3%) had a history of hospital admission.

This study aimed to determine the SARS-CoV-2 antibody seroprevalence in western Iran's general population of Kermanshah province. IgM and IgG antibodies are often developed simultaneously against SARS-CoV-2 in the early stage of the disease; however, those could be detected only in 12.5% of patients on the first day of hospitalization [20]. IgG and IgM levels peak thirty days after the onset of symptoms, then IgM begins to decline, and by the third month, 30% of patients' tests are negative. IgG levels will be high for up to 3 months and drop dramatically in the seventh month [20, 21]. On the other hand, seroconversion depends on several factors, including age, gender, the severity of the disease, and the underlying diseases [22]. This finding suggests that simultaneous detection of IgM and IgG antibodies against SARS-CoV-2 could be a way to improve the diagnosis of infection.

This study is the first population-based serological survey for both active and past SARS-CoV-2 infections in Kermanshah province and has estimated the seroprevalence of IgM or IgG of 18.3 (95% CI: 13.7- 22.7) by late December 2020 (during the third peak). In the present study, a geographical variation was observed in estimating the seroprevalence, which ranged from 2.3 to 61.6%. Our seroprevalence estimate was less than that reported from Semnan (19.3%, 95% CI, 14.0-26.7) in late June 2020 [14] and more than Mazandaran's 15.26% (95% CI: 12.97%-17.79%) in May 2020 [15]. Between April 17th and June 2nd, 2020, in a large and population-based seroprevalence study using obtained sera samples from 5372 individuals with a high risk of exposure to SARS-CoV-2 and 3530 individuals from the general population in 18 cities in 17 Iranian provinces, adjusted seroprevalence was estimated 20% (18.5–21.7) and 17.1% (95% CI 14.6–19.5) ranged from 72.6% (95% CI: 53.9–92.8) to 1.7% (95% CI: 0·0–6·0), respectively [16]. In the above study, the seroepidemiology of the SARS-CoV-2 in Kermanshah city, the capital of the province, in the general population and high-risk groups were 17.3% (95% CI: 5.3-30.9) and 11.9 (95% CI: 7.6-16.4); respectively [16].

A large percentage of Kermanshah residents adhere to the protocols so during the study period, 83% of participants always used masks, 12% occasionally, and only 5% did not use masks. The corresponding values for adherence to physical distancing were 68%, 19%, and 13%. Also, 90% of the participants reported not participating in any gatherings. However, according to the results of this study, almost one-fifth of the residents of the Kermanshah province have experienced COVID-19, which is not enough to achieve herd immunity.

To estimate COVID-19 transmission from asymptomatic individuals, an estimation of the asymptomatic rate seems necessary. We found that almost one-sixth of seropositive individuals did not report COVID-19-related symptoms, and almost 80% did not report contact with an individual with a confirmed COVID-19 infection. The rate of asymptomatic infections ranged from 6.3% to 96.0% in a systematic review [21]. Therefore, seroprevalence studies may be helpful tools for understanding the prevalence of COVID-19 infection in populations where the large majority of individuals are asymptomatic.

We found that the seroprevalence of IgM or IgG among males was 21.9%, which was numerically higher than females with 18.6%. Other studies in Italy [17] and the United Arab Emirates [22] showed that seroprevalence was higher in males than in females. In contrast, a population-based nationwide survey in Iran showed weighted seroprevalence is higher in females than in males (13.2%, 11.1–15.3 vs. 12.5%, 10.5–14.7) [16]. A systematic review and meta-analysis conceded that the seroprevalence does not differ significantly between males and females [18]. However, demographic and social factors may profoundly influence mobility across cultures. Hence higher incidences of SARS-CoV-2 infection may be related to mobility function rather than age and gender. We observed a similar prevalence between individuals in the age group 0-19 and the older age group, which was consistent with the previous reports [19].

Some studies have reported an increased risk of COVID-19 infection among working-aged populations [23, 24]. Though the risk of severe illness or death with COVID-19 increases with age [25], we found the risk of death increase with age. The authors also failed to find evidence that the infection rates differ by education, household size, or comorbidity; However, the infection rates were higher among healthcare workers [26, 27], which was inconsistent with other studies in Iran and the world.

We estimated IFR = 0.3%, which was higher than the rates estimated for South India (0.13%) [24] and the Maranhão in Italy (14%) [28]. Estimating IFR is a challenge as it will depend on the sensitivity and specificity of the test used, age structures, different processes of testing and reporting death, and selective testing of high-risk populations. Therefore, IFR is considerably different across populations.

It is noteworthy that at the time of performing the study, vaccination against COVID-19 had not started in the country, and the first vaccination began on February 20th, 2021, with the priority on medical staff, the elderly population, and high-risk groups. Due to the deficiency of vaccines in Iran, vaccination is slow, and it is challenging to achieve herd immunity through vaccination. However, based on our results, it is estimated that one-fifth of the province's population has been exposed to the SARS-CoV-2 infection, and this number has undoubtedly slightly increased since the survey was completed last December 2020. Although IgG and neutralizing antibodies induced by SARS-CoV-2 infection are protective, several studies have reported subsequent SARS-CoV-2 infection [29, 30]. Nevertheless, Letizia et al. observed that the risk of COVID-19 is five times higher for seronegative young adults than for those with seropositivity [31]. However, the duration of immunity against COVID-19 in seropositive individuals remains uncertain, and protection may decline over time.

This study has several limitations. As mentioned earlier, 25% of those invited were reluctant to participate in the study and were replaced. In addition, data regarding risk factors such as comorbidity, symptom-related COVID-19, history of contact with COVID-19-confirmed individuals and illnesses were based on self-reporting. On the other hand, this study was performed on a large sample size (1967 participants) with high generalizability. Also, participants in all age groups and proportion to size were investigated, which can be the strength of this research.

In conclusion, the prevalence of SARS-CoV-2 infection and IFR in the general population of Kermanshah Province at the end of the third wave of SARS-CoV-2 were 18.3 (13.7- 22.7) and 0.3%; respectively. The results of this study can assist the policymaker in assessing risk factors, transmission dynamics of SARS-CoV-2 in a population, and implementing preventive and control programs.