




































_____________________________________________________________________________________________________ 
 
# 
Professor; 

≡
Director; 

*Corresponding author: E-mail: bawazir56@gmail.com; 
 
 
 

Asian Journal of Immunology 
 
4(1): 146-153, 2021; Article no.AJI.83681 
 

 
 

 

 

Prevalence and Classes of SARS-CoV-2 Antibodies 
among COVID-19 Suspected Patients who Attended 

a Health Care Setting in Sana’a, Yemen 
 

Talal A. Sallam a#, Mokhtar Al-Youssefi b≡ and Amen A Bawazir c*#  

 
a 
Department of Medical Microbiology, Faculty of Medicine and Health Sciences, Sana`a University, 

Yemen. 
b
 Alyoussefi Specialized Laboratories, Sana’a, Yemen. 

c 
Department of Public Health and Epidemiology, Faculty of Medicine and Health Sciences, Aden 

University, Yemen. 
 

Authors’ contributions 
 

This work was carried out in collaboration among all authors. Authors TAS and MAY designed the 
study and  author MAY carried out the data collection. Authors TAS and AAB analysed and 

interpreted the data. All authors read and approved the final manuscript. 
 

Article Information 
 

Editor(s): 
(1) Prof. Cynthia Aracely Alvizo Báez, Autonomous University of Nuevo Leon, Mexico. 

(2) Dr. Wagner Loyola, Brazilian Agricultural Research Corporation, Brazil. 
Reviewers: 

(1) Sadeeq Sheshe, Kano University of Science & Technology, Nigeria. 
(2) Ji Dejun, Yangzhou University, China. 

Complete Peer review History, details of the editor(s), Reviewers and additional Reviewers are available here: 
https://www.sdiarticle5.com/review-history/83681 

 
 
 

Received 19 November 2021  
Accepted 28 December 2021 
Published 29 December 2021 

 
 

ABSTRACT 
 

Aims: This study investigates SARSCoV-2 antibody prevalence and classes among COVID-19 
suspected patients in Sana’a, Yemen. Antibody response to SARS-CoV-2 infection remains to be 
fully elucidated. Currently, no reports on SARS-CoV-2 antibody response from Yemen are 
available.  
Study Design: This cross-sectional study investigates SARS-CoV-2 antibody prevalence and 
classes among COVID-19 suspected patients.  
Place and Duration of Study: This study was conducted in Sana’a the capital of the Republic of 
Yemen from June 2020 through January 2021. 
Methodology: Serological investigation for Anti-SARS-CoV-2 antibody tests was conducted for 259 

Original Research Article 



 
 
 
 

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147 

 

suspected COVID-19 patients who attended a health care facility for antibody testing to confirm the 
diagnosis on C.   
Results: The mean age was 40.8 ±16.6 years. Of all subjects, 180 (69.5%) were males and 79 
(30.5%) were females, 73% were < 50 years of age. A total of 133 (51.4%) had at least one anti-
SARS-CoV-2 antibody class, 6 (2.3%) had isolated IgM, 80 (30.9%) had concomitant IgM and IgG 
and 49 (18.9%) had isolated IgG. Only the seropositivity of isolated anti-SARS-CoV-2 IgG 
significantly (p=0.002) differs among various age groups. There was a significantly higher (p=0.017) 
IgM seropositivity among females than among males.  
Conclusions: Among subjects with suspected COVID-19, > 30% had concomitant IgM and IgG 
with a minority having isolated IgM or IgG suggesting concurrent or close seroconversion time of 
both antibody classes. In addition, around 50% of subjects were SARS-CoV-2 seropositive 
suggestion low SARS-CoV-2 seroconversion and consequently low community seroprevalence. An 
antibody dynamic study based on will characteristics of COVID-19 patients is required. Also, a 
community-based seroprevalence study based on the detection of a combination of IgM, IgG, and 
IgA remains essential to determine the prevalence of SARS-CoV-2 infection in Yemen.  
 

 
Keywords: COVID-19; IgG; IgM; SARSCoV-2; Seroprevalence; Yemen. 
 

1. INTRODUCTION 
 

Immune response to SARS-CoV-2 infection 
remains to be fully elucidated. However, the 
available data, although inconsistent, have 
shown that SARS-CoV-2 infection induces 
antibody response 4 to 15 days post infection[1-
6] with rising levels in severe disease [7,8] and 
increasing seroconversion rates with progress of 
time [2,9,10]. Although anti-SARS-CoV-2 
antibodies response apparently follows 
conventical serological dynamic with sequential 
IgM and IgG, the concomitant appearance of 
both antibody isotypes has been reported [11,12] 
Immunoglobulin M first appears 5-10 days post 
onset of symptoms in most patients, rising about 
2 to 3 weeks rapidly to reach a level maintained 
for 1 to 4 weeks and begin to decline thereafter 
[8,13]. However, IgG antibody has been reported 
to appear 10 days after infection [13], and 
remains detectable up to 6 months after infection 
[14]. Reports on decline time frame of anti-
SARS-CoV-2 antibody levels are inconsistent as 
they vary from 20 [11,12] to 63 days [15-17] for 
IgM and 35 to > 49 days.[8,11,18,19] and even to 
as long as 199 days [18] in the case of IgG. This 
variation in seroconversion time and duration of 
antibodies reflects variations in different studies, 
in patients’ populations, in age and gender of the 
population enrolled, in disease severity, in clinical 
course, and in the serological assays used. 
These together seem to negatively impact the 
attempt to reliably establish the dynamic of the 
antibody response and hinder a genuine 
estimate of seroprevalence and the herd 
immunity level.  
 

So far epidemiological studies have shown 
conflicting low seroprevalence rates of SARS-

CoV-2 antibodies, in different parts of the world. 
Various reports have shown seroprevalence 
rates of 4 to 7.3% in several part of Europe, 4.2% 
in Northern America, 22% in Central and 
southern Asia [17], 0.07% in South Korea [16], 
3.8% in Wuhan geographic regions [19] and 
7.8% in Malaysia [20]. This variation suggests 
varying time of testing through the pandemic, 
demographic and geographic variations and 
variation of serological assays used. The low 
seroprevalence rates is attributable to the fact 
that majority of the studies were mainly based on 
detection of IgG and IgM whereas IgA was found 
in COVID-19 patients in isolation of IgM and IgG 
[20,21]. Thus inclusion of IgA detection in 
seroprevalence studies detects more COVID-19 
infections. Yemen is one of the countries where 
confinements were inadequately implemented 
during spread of COVID-19 because of the 
current political turmoil and the collapse of the 
health care system. Based on latest estimate of 
the Ministry of Public Health and Population in 
Yemen, COVID-19 might expand and potentially 
infect 90% of the population [2,22]. Despite this 
gloomy scenario the other side of the story is 
vowing as the absence of containment measures 
will promote the development of natural herd 
immunity.  
 

Despite the frequent reports of SARSCoV-2 
antibody prevalence and insights on antibody 
response from a number of countries around the 
globe but not from Sana’a, Yemen. Yemen as 
one of the countries where confinement of 
COVID-19 was ineffectively implemented since 
the emergence of the disease because of the 
current political turmoil and the collapse of the 
health care system. This is one of the very few 
studies if not the first on antibody response and 



 
 
 
 

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148 

 

seroprevalence of SARS-CoV-2 in the major city 
of Sana`a, northern part of Yemen. Moreover, 
this study will provide insights on the antibody 
response to SARS-CoV-2 in Yemen where the 
study was conducted relatively early during 
SARS-CoV-2 pandemic as well as it could work 
as a baseline to further studies that measure the 
level of herd immunity to COVID-19 in the 
country. Therefore, the aim of this study was to 
investigate the antibody response and to 
estimate the seroprevalence of SARS-CoV-2 
among COVID-19- suspected patients who 
attended a health care setting for COVID-19 
testing in Sana`a, Yemen.  
 

2. MATERIALS AND METHODS  
 

2.1 Study Design and Settings 
 
This cross- sectional study was conducted 
retrospectively in the Sana`a region in Yemen 
during the second week of June 2020 through 
January 2021. On an informed consent the anti-
SARS-CoV-2 antibody tests results and the 
demographic data that included age and gender 
of 259 COVID-19 suspected patients who were 
referred to a health care facility for antibody 
testing to confirm the diagnosis were 
anonymously enrolled in this study.   
 

2.2 Antibody Testing 
 

The sera were tested for anti-SARS-CoV-2 IgM 
and IgG antibodies using iFlash –SARS-CoV-2 
IgM and IgG paramagnetic particle 
chemiluminescent (CLIA) (Shenzhen yhlo 
biotech co., ltd. China) for qualitative 
determination of IgM and IgG according to the 
manufacture instructions. In summary prediluted 
serum samples were incubated with a 
recombinant SARS-CoV-2 antigen that was 
coated to paramagnetic microparticles to allow 
the formation of antigen- antibody complex. Then 
complex was washed under magnetic field during 

which magnetic particles were absorbed into the 
inner wall of the reaction tubes while the 
unbound materials were washed away from the 
solid phase of the magnetic field. Then the 
complex was incubated with Acridinium-labeled 
anti-human antibody conjugate. The newly 
formed complex that consisted of SARS-CoV-2 
antigen- antibody and acridinium-labeled anti-
human antibody was washed. Then a pre-trigger 
and a trigger solution were added to trigger the 
signal and the resulting chemiluminescent 
reaction was measured as relative light units 
(RLUs) the detection of which by the iFlash 
optical system was proportional to the antibody 
concentration present in the sample. The results 
are determined through a calibration curve by 2-
point calibration. Antibody level ≥ 10 Absorbance 
units per milliliter (AU/ml) was considered 
positive.  
 

2.3 Statistical Analysis 
 

Data entered in a statistical package (SPSS 
version 22) for analysis. Categorical variables 
were used for the description of the demographic 
characteristics of the participants (age and sex). 
Association between the seroprevalence findings 
of the IgM and IgG with the characteristics of the 
participants were undertaken using a chi-square 
test, where p value findings of < 0.05 was 
considered significant.  
 

3. RESULTS 
 

The mean age of the 259 subjects was 40.8 
±16.6 years, ranging between 3-85 years. Most 
of the participants 180 (69.5%) were males and 
79 (30.5%) were females, young adults (≤18 -49 
years), who were mostly infected in week 2 of 
June 2020 (Fig. 1). A total of 133 (51.4%) were 
positive for at least one anti-SARS-CoV-2 
antibody class, and of these 6 (2.3%) had 
isolated IgM, 80 (30.9%) had concomitant IgM 
and IgG, 49 (18.9%) had isolated IgG, and 128 
(49.4%) had IgG with or without IgM (Table 1). 

 
Table 1. Seropositivity rate of various anti-SARS-CoV-2 antibody classes among COVID-19- 

suspected subjects who attended a health care facility in in Sana`a, Yemen 
 

Variable Category n % 

Antibody response At least one Antibodies 133 51.4 
 Absence of antibodies 126 48.6 
 Isolated IgM 6 2.3 
 Concomitant IgG/IgM 80 30.9 
 Isolated IgG 49 18.9 
 IgG with/without IgM 128 49.4 

 



 
 
 
 

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149 

 

Table 2. Seropositivity rate of anti-SARS-CoV-2 antibody classes among males and females and different age groups of COVID-19- suspected 
subjects who attended a health care facility in in Sana`a, Yemen(n=259) 

 

Variable At least 1 antibody 
class n= 133 

Isolated IgM (n= 6) Isolated IgG (n= 49) IgG with/without IgM 
(n=128) 

Concomitant IgM & 
IgG (n= 80) 

n (%) p n (%) p n (%) p  n (%) p n (%) p 

Sex Male 88 (69.8) 0.907 1 (17.0) 0.017 144 (68.6) 0.503 89 (67.9) 0.581 125 (69.8) 0.861 
 Female 38 (30.2)  5 (83.0)  66 (31.4)  42 (32.1)  54 (30.2)  
Age (years) 3-18  8 (6.3) 0.069 0 (0.0)  0.721 16 (7.6) 0.002 8 (6.1) 0.112 8 (4.5) 0.074 
 19-49  85 (67.5)  5 (83.3)  147 (70.0)  89 (67.9)  117 (65.4)  
 50-64  13 (10.3)  1 (16.7)  23 (11.0)  14 (10.7)  29 (16.2)  
 ≥ 65  20 (15.9)  0 (0.0)  24 (11.4)  20 (15.3)  25 (14.0)  

 

 
 

Fig. 1. Demographic characteristics of subjects COVID-19 -suspected who attended a health care facility in in Sana`a, Yemen (n=259) 

69.5 

30.5 

6.2 

67.2 

14.7 
11.9 

0 

10 

20 

30 

40 

50 

60 

70 

80 

Male Female 18-Mar 19-49  50-64  ≥ 65  

Sex Age (years) 

P
er

ce
n

ta
g
es

 



 
 
 
 

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3.1 Seropositivity Rate According to 
Antibody Classes by Age and Sex 

 
Although, the seropositivity rate of the isolated 
anti-SARS-CoV-2 IgG significantly (p=0.002) 
differs among various age groups, the 
seropositivity of the IgG and the IgM in 
combination or in isolation did not differ 
significantly (p>0.05) between males and 
females or between various age groups with the 
highest seropositivity rate detected being among 
the 19-49 years age group and the lowest 
detected was among ≥65 years age group (Table 
2).  
 

4. DISCUSSION  
 
To the best of our knowledge, this is one of very 
few studies if not the first on seroprevalence of 
SARS-CoV-2 in the major city of Sana`a, 
northern part of Yemen. Of all patients who were 
suspected of having COVID-19 around half of 
them had at least one antibody marker, either 
IgG or IgM or both, suggesting exposure to 
SARS-CoV-2 infection. This is obviously because 
these patients were not clinically well 
characterized as cases of COVID-19 but were 
referred for testing on suspension of the disease 
having presented with suggestive signs and 
symptoms. This explain the lower 
seroprevalence than has been reported among 
well characterized patients who were clinically 
diagnosed with COVID-19 early in epidemic 
where almost 100% became antibody positive 2 
to 3 weeks post disease onset [7,12]. However, 
owing to the attendance of the majority of the 
cases in a single health care facility for testing in 
only two weeks’ time indicating the peak of 
epidemic, the contagious nature of COVID-19 
and the lack of adequate confinement measures, 
a higher seropositive can be expected. One 
reason for the low seroprevalence rate is that the 
subjects may have produced transient systemic 
antibodies because of less severe or mild 
infection. It has been reported that systemic 
antibody production against SARS-CoV-2 
develops mainly in severe COVID-19, whereas 
mild disease may be associated with transient 
serum anti- SARS-CoV-2- specific antibodies [9]. 
Additionally, our subjects may have responded 
with more localized respiratory antibody 
response. It has been reported that some SARS-
CoV-2-exposed cohorts were negative for SARS-
CoV-2-specific serum IgA and IgG but showed 
detectable SARS-CoV-2-specific IgA in nasal 
fluids and tears without serum antibody response 
[9]. Additionally symptomatic patients who may 

not seroconvert despite developing persisting T 
cell responses have also been reported [7]. 
Several reports have shown underestimated 
seroprevalence when investigation was based on 
IgM and IgG alone [20]. 

 
Over 70% of our seropositive subjects were 
below the age of 50 years, which reflects the 
main age stratum of the Yemeni population (< 50 
years) constituting 90% of the population [23], 
rather than suggests that this age groups were 
particularly susceptible. The low seroprevalence 
among those 3-18 years may reflect the mild or 
asymptomatic infection that usually occur in this 
age group and thus do not seek health care. 
Various reports have shown that less children 
(<18 years) than adults presented with mild to 
less severe and lower death rate [24-26]. The 
attendance of small proportion of our subjects of 
this age for testing further supports this 
inference. The low level of SARS-CoV-2 infection 
among children has been explained by the less 
expressed angiotensin-converting enzyme 2 
(ACE2) receptor of the virus, better containment 
of the virus in the upper tract or to the less 
adverse immune mechanisms arising from 
infection in <18 years than in adults [22] . 

 
Among our subjects less females than males 
presented for testing which could suggests low 
exposure among females. However this possibly 
because of stay-at-home custom of Yemeni 
women rather than due to low susceptibility to 
infection among females. Supporting this is the 
seroprevalence rate among females which did 
not significantly differ from that among males 
indicating equal risk of infection. Small minority of 
seropositive subjects (2.3%) had an isolated IgM 
and 18.9% had isolated IgG while over 30% had 
concurrent IgM and IgG. This could suggest the 
narrow window period of the antibody class 
switching from IgM to IgG making it difficult to 
detect patients with isolated IgM and to less 
extent isolated IgG. The possibility of 
simultaneous occurrence of both antibody 
classes cannot be excluded as simultaneous 
appearance of both antibody classes in some of 
SARS-CoV-2 infected subjects has been 
reported elsewhere [7,9,10,12-22,25-31] 
Furthermore, 3 patterns of SARS-CoV-2 
seroconversion have been proposed so far. 
These include sequential seroconversion of IgM 
followed by IgG, concurrent seroconversion of 
both IgG and IgM, and an unusual pattern of IgG 
preceding IgM [30]. This probably reflects the 
variation in different serological assays. 
Therefore, validation and standardization of 



 
 
 
 

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151 

 

SARS-CoV-2 serologic assays in large clinical 
cohort is required before coming to a final 
inference of serological dynamic of SARS-CoV-2. 
Majority of our subjects with isolated IgM were 
females (83%). The significance of this is difficult 
to point out due to the small sample size (6 
subjects). However, this merit further 
investigation.  
 

5. CONCLUSIONS 
 

Among subjects with suspected COVID-19 over 
30% had concomitant IgM and IgG with minority 
having isolated IgM or IgG suggesting 
concomitant or close seroconversion time point 
of both antibody classes. In addition, around 50% 
of subjects were SARS-CoV-2 seropositive 
suggestion low SARS-CoV-2 seroconversion and 
consequently low community SARS-CoV-2 
seroprevalence in Yemen. As the systemic 
immune response to SARS-CoV-2 currently is 
not well established, the seroprevalence of 
SARS-CoV-2 cannot be reliably determined 
hence herd immunity seems to be hard to 
establish at this stage. A large-scale community- 
based seroprevalence study based on detection 
of combination of IgM, IgG, and IgA seems 
essential to determine the magnitude of SARS-
CoV-2 infection in Yemen. Antibody response 
study based on will characterized COVID-19 
patients is also required.  
 

CONSENT 
 

Data presented in this study were obtained in an 
informed consent with a clear declaration on the 
confidentiality of the obtained data and will not be 
used out of the research dimension. 
 

ETHICAL APPROVAL  
 

This study attested and approved (# REC-77-
2020) by relevant academic institution. 
 

ACKNOWLEDGMENTS 
 

The authors are grateful to Mr. Mohammad 
Bamashmoos at the Alyoussefi Specialized 
Laboratories for his efforts in the data collection.  
 

COMPETING INTERESTS 
 

Authors have declared that no competing 
interests exist. 

 

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