Determinants of having severe acute respiratory syndrome coronavirus 2 neutralizing antibodies in Egypt

Abstract Background Reported laboratory‐confirmed COVID‐19 cases underestimate the true burden of disease as cases without laboratory confirmation, and asymptomatic and mild cases are missed by local surveillance systems. Population‐based seroprevalence studies can provide better estimates of burden of disease by taking into account infections that were missed by surveillance systems. Additionally, little is known about the determinants of seroconversion in community settings. Methods We conducted a cross‐sectional serologic survey among 888 participants in Egypt. Results Neutralizing antibodies were detected in 30% of study volunteers. Age and educational level were associated with being seropositive as people older than 70 years and people with graduate degrees had lower seroprevalence. Self‐reporting cases having COVID‐19‐related symptoms such as fever, malaise, headache, dyspnea, dry cough, chest pain, diarrhea, and loss of taste or smell were all associated with having antibodies. Fever and loss of taste or smell were strong predictors with odds ratios of 2.1 (95% confidence interval: 1.3–3.5) and 4.5 (95% confidence interval: 2.6–7.8), respectively. Conclusions Our results can guide COVID‐19 prevention and control policies and assist in determining the immunity level in some Egyptian communities.


| INTRODUCTION
As of December 29, 2020, the world had more than 79 million cases of COVID-19 of whom more than 1.7 million died. 1 The number of cases represents laboratory-confirmed cases and is curated by the World Health Organization (WHO) based on official reports from participating countries and territories. This figure underestimates the true burden of disease as cases without laboratory confirmation as well as asymptomatic and mild cases that are missed by local surveillance systems are not reported. Consequently, the case fatality rate (CFR), 2 | METHODS

| Design and study population
The study population was employees of a major research institution in Egypt located in Greater Cairo, totaling around 7000. 9 In the period August-October 2020, study personnel visited all departments of the institution on two different days and invited all individuals present at the time of the visit and who never had a laboratory-confirmed SARS-CoV-2 infection to participate. Upon completing the informed consent process and signing the consent form, subjects were asked a series of demographic, health status, and COVID-19 exposure questions using a specifically tailored questionnaire. A phlebotomist then collected a 3-ml blood sample for serum collection. Sera were separated by centrifuging blood at 1000 g for 15 min. All samples were heat inactivated at 56 C for 30 min and stored at À20 C until testing.

| Microneutralization assay
Sera were tested for neutralizing antibodies using a microneutralization assay (MN). The MN was conducted as described previously using Vero-E6 cell monolayers. 10 Briefly, serial twofold dilutions of heatinactivated sera starting with a dilution of 1:10 were mixed with equal volumes of 100 tissue culture infectious dose (TCID50 per milliliter) of hCoV-19/Egypt/NRC-03/2020 SARS-CoV-2 isolate. After 1 h of incubation at 37 C, 35 μl of the virus-plasma mixture was added in duplicate to Vero-E6 cell monolayers in 96-well microtiter plates. After 1 h of adsorption, the inoculums were aspirated. The plates were then incubated for three more days at 37 C in 5% CO 2 in a humidified incubator. A virus back-titration was performed without immune serum to assess input virus dose. Cytopathic effect (CPE) was read at 3 days post infection (dpi). The highest serum dilution that completely protected the cells from CPE was recorded as the neutralizing antibody titer. Sera testing negative at a 1:10 dilution were given a nominal value of 1:5.

| Statistical analysis
SPSS v23 (IBM, Armonk NY) was used for analysis. Chi-square was used to compare seropositivity rates within categorical variables. P value < .05 was considered statistically significant. Logistic regression was used to calculate adjusted odds ratios using all variables that were significant in bivariate analysis.

| RESULTS
A total of 888 subjects participated in this study. Demographic and health data of study participants are shown in Table 1 Around 39% of the participants reported having influenza-like illness (ILI) symptoms in the 3 months preceding enrollment in the study (Table 2). Most common symptoms were malaise, sore throat, rhinitis, fever, headache, and dry cough. Fifteen percent of subjects reported being exposed to a COVID-19 patient. This exposure was mainly at work (40.6%) or within the subject's household (34%). Among the participants, 37.8% reported attending a social gathering, and 24.2% reported domestic or international travel in the 3 months preceding enrollment. The majority of the participants (91.3%) reported using face masks; 22.9% reported using public transport. Fifty-eight subjects (6.5%) were healthcare workers involved in direct patient care.
Determinants of seropositivity are shown in Table 3. Age was significantly associated with antibody levels (P value = .039 Despite this, population-based seroprevalence studies in lessdeveloped countries are sparse. In a serosurvey conducted in Brazzaville, Congo, in the period April-July 2020, anti-SARS-CoV-2 antibodies were detected in 15% of the sampled adults. 13 In Guilan region of Iran, 22% of study participants enrolled in April 2020 had antibodies. 14 A larger national study in Iran conducted between April and June 2020 reported a 17% seroprevalence rate. 15 A study among migrant workers in Kuwait revealed a 38% seroprevalence rate during May-June 2020. 16 The seroprevalence rate among Kenyan blood donors was estimated at 4.3% during April-June 2020. 17 In comparison with the above-mentioned reports, our study is the only one that relied on neutralizing antibodies to determine seroprevalence of SARS-CoV-2 antibodies. Hence, detected antibodies are only those that directly neutralize the virus rendering it noninfectious and nonpathogenic. This means that our reported seroprevalence is lower than what would have been reported if we used assays that cover a broader range of antibody types. The overall detected seroprevalence rate is 30%, higher than those reported from other studies.
If a more conservative cutoff titer of ≥1:40 is used, the seroprevalence rate becomes 22%. Despite the relatively high seroprevalence rate, none of the participants had laboratory-confirmed COVID-19, but a proportion of them reported having respiratory symptoms. This could potentially indicate that the majority of those who seroconverted had asymptomatic or mild disease. 18 The reported seroprevalence may have been higher had we included individuals with previously confirmed COVID-19.
Age was associated with seropositivity in our study as all age categories, except for those older than 70, had a seropositive proportion.
T A B L E 3 (Continued)  19 This discrepancy can be explained by differences in the population structure between participants in the two studies, COVID-19 spread differences in the two countries, or different behaviors in the two populations.
Having completed graduate studies or being a faculty member was protective against being seropositive. This indicates that more educated people tend to adhere to protective measures more than others and hence are better protected against COVID-19. Having a more senior healthcare profession was found to be protective against being seropositive in a study conducted in the United Kingdom. 20 Self-reporting of having respiratory symptoms was a predictor of having antibodies. A meta-analysis of seroprevalence studies among healthcare workers showed the same finding as our study. 21 Several self-reported symptoms were associated with seropositivity, but the most profound was reporting of loss of taste or smell. This association remained significant even after adjusting for other variables. This finding is supported by a population-based study in the Netherlands and by studies among healthcare workers globally. 19,21 Fever and dyspnea were also predictors of seropositivity as indicated by other studies. 19 Anosmia and ageusia appear to be a hallmark of COVID-19. Modifiable and behavioral variables such as wearing personal protective equipment or avoiding travel or gatherings were not statistically associated with having antibodies.
Our findings are not generalizable to the general Egyptian population due to potential selection bias. The study was conducted among employees of a single institution with a high proportion of welleducated individuals. However, better-educated participants had lower seroprevalence. Hence, our results may be underestimating the true prevalence in the general population. It is unlikely that this bias affected our findings related to determinants of seroprevalence. To properly estimate the seroprevalence and its determinants in the entire population, a larger national study is required. Furthermore, our findings support the notion that the number of reported COVID-19 cases is severely underestimated due to the commonality of asymptomatic and mild disease. In contrast, Egypt reported 107 555 cases by the end of October 2020, around 0.1% of the Egyptian population.
Another limitation is that the study relied on a convenience sample rather than a randomly selected sample as the institution was functioning at a reduced workforce level during the study period due to the pandemic.
In summary, our study is among the few to measure determinants of SARS-CoV-2 antibodies especially in a region that had a relatively less severe burden of COVID-19. Our results can guide COVID-19 prevention and control policies and assist in determining the immunity level in some Egyptian communities. For instance, vaccines can be directed to communities or age groups with lower immunity levels while maintaining more strict nonpharmaceutical interventions in such groups.