SQU Med J, February 2011, Vol. 11, Iss. 1, pp. 77-82, Epub. 12th Feb 11
Submitted 15th Jun 10
Revision ReQ. 18th Jul 10, Revision recd. 26th Jul 10
Accepted 11th Aug 10

1Department of Medical Microbiology, Faculty of Medicine & Health Sciences, Sana’a University, Yemen; 2Department of Medical 
Microbiology, Faculty of Medicine & Health Sciences, Taiz University, Yemen; 3Department of Medical Microbiology, Faculty of 
Medical Sciences, University of Sciences & Technology, Sana'a, Yemen.
*Corresponding Author email: shmahe@yemen.net.ye

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
 840           

         %44 %56504
 
%69.9
504 276%66.8%72.1
 10           %54.8
%45.2504228
 109 %63.6 53





aBStract: Objectives: The study was undertaken, first, to determine the coverage rate of hepatitis B (HB) vaccine 
and second to evaluate the immune response to HB vaccine among children under 10 years old by measuring the 
level of circulating anti-HB surface antigen (anti-HBs) antibodies after immunisation with three doses. Methods: 
First, 840 children were randomly selected from 4 randomly selected sites in Sana'a city to study the coverage 
rate of the vaccine; of these, 504 children vaccinated against HBV prior to the study, were tested  (56% males and 
44% females). Sera were tested for anti-HBs antibodies by ELISA quantitative technique. Each individual’s data 
was collected in a pre-designed questionnaire including: vaccination date, sex, and age at the time of the study. 
Results: The coverage rate of HBV vaccine was only 69.9%, being slightly higher among male children (72.1%) 
than female children (66.8%). A total of 276 (54.8%) of the 504 children responded to the vaccine with anti-HBs 
antibody level ≥ 10 mIU/ml, while 228 (45.2%) of the 504 children had non-protective anti-HBs antibodies levels 
(<10IU/ml). Children of ages 3–5 years had the highest protective rate (63.6%), and the lowest protective rate 
was in the 9–10 years age group. Conclusion: This study revealed a low coverage rate of HBV vaccine and a low 
protective rate against HBV infection. A considerable proportion of vaccinated children should be considered for 
either revaccination or booster doses. There is also the need to complete HBV vaccine coverage among the child 
population in San’a, Yemen.

Keywords: Hepatitis B; Vaccine; Children; Yemen; Sana'a

Hepatitis B Vaccine Coverage and the 
Immune Response in Children under 

ten years old in Sana’a, Yemen
*Hassan A Al-Shamahy,1 Samira H Hanash,2 Iqbal A Rabbad,3 Nameem M Al-Madhaji,1 Samarih M Naser1

CLINICAL & BASIC RESEARCH

 ADVanceS in KnoWleDGe
1.  There is a low coverage rate of hepatitis B (HB) vaccine in Sana’a, Yemen (70% of children) as well as a low protective rate against HB 

infection. This is despite the existence of an official vaccine supply since 2000.



Hepatitis B Vaccine Coverage and the Immune Response in Children under ten years old in Sana’a, Yemen

78 | SQU Medical Journal, February 2011, Volume 11, Issue 1

Hepatitis B (HB) is a human disease caused by a virus that attacks the liver. The hepatitis B virus (HBV) can cause 
lifelong infection, cirrhosis of the liver, liver cancer, 
liver failure and death.1 HB infection is one of 
the world’s most common and serious infectious 
diseases. It is estimated that more than one third of 
the world’s population has been infected with HBV.2 
About 5% of the population are chronic carriers of 
HBV, and nearly 25% of all carriers develop serious 
liver diseases such as chronic hepatitis, cirrhosis 
and primary heptocellular carcinoma (HCC).3 HBV 
infection causes more than one million deaths every 
year.4 

For long-term protection against HBV, there 
are two types of vaccines: plasma-derived HB 
surface antigen (HBsAg) vaccine, and yeast-derived 
HBsAg vaccine.5 HB immunisation, using either 
type of vaccine, has been shown to eliminate HBV 
transmission and prevent HBV-related chronic 
liver disease.6 HBV vaccine can be routinely given 
to children and individuals at risk, along with other 
commonly used vaccines  in a variety of schedules 
that results in excellent immunogenicity and do 
not interfere with the immunogenicity of other 
vaccines.7  The sero-conversion rate for vaccination 
is influenced by a number of factors, the most 
important ones being age and sex. Rates in excess 
of 95% are seen in young women, whereas the rate 
may drop to 80% in older men. Immunosuppressed 
patients, smokers, and obese individuals show even 
lower rates.8 

According to the Yemeni National Infectious 
Viral Hepatitis Control Programme, Yemen was 
recognised as HBV-endemic area.9 In 1998, the 
World Health Organization (WHO) recommended 
the inclusion of HB vaccine in the national 
immunisation programme of Yemen, particularly 
among neonates, where vertical transmission is 
common, regardless of the prevalence of HBsAg. 
In Sana’a city, the HB antigen carrier rate is 
classified as intermediate, rather than high. The 
incidence of acute HBV has declined dramatically 
in the past decade since the start of the vaccination 
programme, especially among young people, 

although it may still take several decades until the 
effect of vaccination is translated into reduced 
transmission and morbidity.10

The aims of this study were first to determine the 
coverage rate of HBV among children and second 
to evaluate the immune response to HBV vaccine 
among vaccinated children with the three doses of 
HBV vaccine by measuring the level of circulating 
anti-HB surface antibodies. 

Methods 
This cross sectional study was carried out from 
January to March 2010. The study proposal 
was approved by the Department of Medical 
Microbiology, Faculty of Medicine & Health 
Sciences, Sana’a University, Yemen. First, 840 
children, aged <1–10 years, (470 males and 370 
females) were randomly selected by a systematic 
random sampling of every fifth child attending 
randomly selected health centres or from the lists 
of randomly selected primary schools in Sana’a city. 
The histories of all 540 children who had received 
three doses of HBV vaccine were investigated. A 
total of 81.9% of them had received yeast-derived 
vaccine and 18.1% plasma-derived vaccin; 62.2% of 
the children had been vaccinated at a dose interval 
of 0, 1, 2 months  and 37.8% at a dose interval of 
0, 1, 6 months  The immune response to the HBV 
vaccine of these 504 vaccinated children was then 
evaluated. A consent form was completed by the 
parents for each participant.

A full history was taken from each individual 
studied and the findings recorded in a pre-designed 
questionnaire. The data collected included name, 
age at the time of the study, sex, residence, and 
vaccination date according to the last dose of HBV 
vaccine, number of doses, intervals between the 
three doses and type of vaccine. A full history of 
vaccinations against other diseases was also taken.

Four millilitres of whole blood were collected 
from each subject. Then the sera were separated 
and frozen at -20 °C until tested. Anti-HBs 
antibodies were determined by an enzyme-
linked immunosorbent assay (ELISA) using a 

Application to Patient Care
1.  Health care centres need to pay more attention to boosting HB vaccine coverage among children.
2.  The role of vaccine in HB protection is fundamentally important as HB is endemic in Sana’a.



Hassan A Al-Shamahy, Samira H Hanash, Iqbal A Rabbad, Nameem M Al-Madhaji and Samarih M Naser

Clinical and Basic Research | 79

commercially available kit (Biokit, S.A., Barcelona, 
Spain). Bio-ELISA anti-HB is a direct immuno-
enzymatic method of the “sandwich” type in which 
the samples to be analysed are incubated in wells of 
micro-plate that are coated with highly purified HB 
antigens. 

In order to differentiate naturally occurring 
immunity to HBV infection, total anti-HB core 
antibodies were measured; all individuals with anti-
HB core antibody positive were excluded from the 
evaluation of HBV vaccine group.

   The data and results were analysed by using 
EPI-Info Version 6, Centers for Disease Control 
(CDC, Atlanta, GA, USA).

Results
The coverage rate of HBV vaccine was only 69.9%, 
slightly higher among male children (72.1%) than 
females (66.8%) [Table 1].

Table 2 shows the immune response to HBV 
vaccine by quantifying anti-HB antibody levels 
among males and females. A total of 276 (54.8%) of 
the 504 children responded to the vaccine with anti-

HBs antibody levels ≥10 mIU/ml, while 228 (45.2%) 
had non-protective anti-HBs antibodies level 
(<10 IU/ml). The average protective rate in both 
sexes was 54.8%, but higher among males (57.4%) 
than females (51.4%). There was no statistically 
significant variation between both sexes. Table 
3 shows the protective rate among different age 
groups, the rate being higher in younger compared 
to older age groups. Children aged 3–5 years had 
the highest protective rate (63.6%); the lowest 
protective rate was in the 9–10 year old age group 
(32.4%) [Table 3].

Table 4 shows the protection by yearly intervals 
after primary immunisation against HBV. Children 
immunised one year or less prior to the study 
had a higher protective rate (66.7%), than those 
immunised 4 years prior (44.2%). 

In this study, different HBV-markers were 
obtained from the whole 504 group of vaccinated 
children. It was found that the frequency of HBsAg 
positivity among these children was 1.8%.

Table 1: The coverage rate of hepatitis B vaccine (HBV) among randomly selected male and female children under 
ten years old in Sana’a, Yemen

Status
         Male (n = 470)       Female (n =370)           Total (n =840)

X2 P value
No. % No. % No. %

Vaccinated for HBV 339 72.1 247 66.8 586 69.9

2.83 0.09Non-vaccinated for HBV
Total

131
470

27.9
56

123
370

33.2
44

254
810

30.2
100

Table 2: The immune response to hepatitis B vaccine by quantifying anti-HBs antibody levels and protective and non-
protective levels among males and females

Antibody levels Male % Female % Total % χ2 P value*

Non-responders        
(<10 mIU/ml)

120 42.6 108 48.6 228 54.1 1.8 0.17

Low-responders    
(10–100 mIU/ml )

87 30.9 45 20.3 132
26.2

7.2 0.007

Adequate responders 
(>100–1000 mIU/ml )

45 16 45 20.3 90 17.6 1.6 0.2

High-responders
(>1000 mIU/ml )

30 10.6 24 10.8 54 10.7 0.0 0.95

Immune response 

Protective anti-HBs
Non-protective anti-
HBs

162
120

57.4
42.6

114
108

51.4
48.6

276
228

54.8
45.2

1.9 0.17

Notes: * χ2≥ 3.84, P <0.05 (significant); Protective anti-HBs ≥10 mIU/ml; Non-protective anti-HBs <10 mIU/ml.



Hepatitis B Vaccine Coverage and the Immune Response in Children under ten years old in Sana’a, Yemen

80 | SQU Medical Journal, February 2011, Volume 11, Issue 1

Discussion
Since the 1980s, there has been an increasing body 
of information on viral hepatitis in Yemen, which is 
a major public health problem affecting thousands 
of people throughout the country.10 Viral hepatitis is 
a major cause of morbidity and mortality in humans 
in Yemen, both from acute infection and its chronic 
sequelae which include hepatitis B and hepatitis C 
infection, chronic hepatitis cirrhosis and primary 
liver cancer.11 The endemic rate of hepatitis B virus 
infection is considered high in Yemen, where the 
prevalence of the positive (HBsAg) ranges from 
8–20%, and up to 50% of the general population have 
serological evidence of previous HBV infection.12

Yemen introduced a universal immunisation 
programme against HBV for infants and high 
risk groups in early 2000, but feed-back on the 
coverage rate of vaccination and its efficacy in the 
community have been ignored for a long period. 
In addition, there has been inadequate information 
on the prevalence and risk determinants of viral 
hepatitis as well as on vaccination coverage rate 
among children in Yemen. This study was carried 
out in response to this information gap.

One of the aims of this study was to determine 
the coverage rate of HBV vaccine among children. 
The study findings showed that the vaccination 
coverage rate was 69.9%. This result is lower than 
findings in other HBV endemic countries, where 
HBV vaccine coverage rates among children range 
from 90–98%.13-15 

Also the study findings showed that only 54.8% 
of all vaccinated individuals were regarded as 
protected (≥10 mIU/ml), and that the protective rate 
of HBs antibody was higher in males (57.4%), than 
in females (51.3%). Different findings were reported 
elsewhere among children, where a high protective 
anti-HB response rate was found among vaccinated 
children (97.4%), and the rate for females was also 
higher than that for males.16,17 This difference in the 
findings could be attributed to a different response 
in the primary course of vaccination, different age 
groups, to the different degrees of exposure to 
natural boosters, or to differences in nutritional 
status and socioeconomic factors, race factors, or 
the type of vaccines used.18

Concerning the rest of the study group, 45.2% 
developed a low antibody level (<10 mIU/ml), 
indicating a poor anti-HBs response after receiving 

Table 3: Immune response among vaccinated children in different age groups

Age groups
Protected Non-protected

X2 P value
No. % No. %

1–2 years (n = 72) 39 54.2 33 45.8 0.01 0.91

3–5years (n = 165) 105 63.6 60 36.4 7.8 0.005

6–8years (n = 165) 99 60 66 40 2.7 0.09

9–10 years (n = 102) 33 32.4 69 67.6 25.9 0.0000004

Total 276 54.8 228 45.2
Notes: Protected (anti-HBs ≥10 mIU/ml); Non-protected (anti-HBs <10 mIU/ml).

Table 4: Protected and non-protected vaccinated individuals according to period since vaccination

Year intervals

Protected 
(n = 276)

Non-protected 
(n = 228) X2 P value

No. % No. %

1 year or less prior to study (n = 72) 48 66.7 24 33.3 4.8 0.02

2 years prior to study (n = 125) 78 62.4 47 37.6 3.9 0.04

3 years prior to study (n = 119) 69 57.9 50 42.1 0.65 0.41

4 years prior to study (n = 86) 38 44.2 48 55.8 4.7 0.03

5 years or more prior to study (n = 102) 48 47.1 54 52.9 3.1 0.08

Total no. = 504
Notes: Protected ≥10 mIU/ml; Non protected <10 mIU/ml.



Hassan A Al-Shamahy, Samira H Hanash, Iqbal A Rabbad, Nameem M Al-Madhaji and Samarih M Naser

Clinical and Basic Research | 81

a full course of vaccine, as shown in Table 2. It 
can be deduced from this finding either that these 
vaccinated individuals were hypo-responsive to 
the immunisation and that their antibodies may 
have waned rapidly over time, or that the vaccine 
was of poor quality. Even in these instances, loss 
of antibodies does not necessarily imply loss 
of protection.19 Considering that anti-HBs may 
disappear in a substantial proportion of vaccinees 
after initially successful vaccination, a booster 
dose of vaccine, following the administration of 
the primary course, is recommended by most 
national bodies. However, the results of long-term 
follow-up studies, together with assessment of the 
role of immunological memory among vaccinees, 
now call into question the necessity of providing 
booster doses following successful course of 
primary immunisation.19 Other studies showed 
that protection is still maintained among vaccinees, 
even in HBV-endemic countries, despite waning or 
undetectable anti-HBs levels.20-23

In this study, different HBV-markers were 
obtained from all the vaccinated children studied. 
However, due to the lack of serological data, either 
before or after vaccination, it was impossible to 
conclude whether these children were already 
infected at the time of vaccination or had been 
infected subsequently. In the present study, it was 
found that the frequency of HBsAg positivity among 
the whole group of children was 1.8%, which was 
lower than the rate of the non-vaccinated children 
(2.8%) in Sana’a city in 2001.11 This indicates the 
efficacy of HBV vaccine in preventing chronic 
carriage of infection. In a long-term follow-up study 
(over c. 16 years) on HB vaccine immune efficacy 
in China, the positive rate of HBsAg for children 
born after the introduction of the immunisation 
programme was much lower than those of the 
background group before vaccination.24,25 Also our 
result were similar to that found in Egypt among 
children, where HBsAg positivity was 0.8% in 
vaccinated children compared to 2.2% for non-
vaccinated children.26 No clinically overt hepatitis 
has been reported so far among the studied 
vaccinated individuals. This was similar to findings 
reported elsewhere.27,28 

Host factors, such as age, may influence the 
immune response to the vaccine.29 Increasing age 
was shown to be correlated with a decreasing level 
of protection rate [Table 3]. The response rate of 

anti-HBs declined from 63.6% in the 3–5 years age 
group to 32.4% in the 9–10 years age group. Similar 
findings were reported from Saudi Arabia, showing 
that being in the >10 years age group correlated 
with a decreasing protection rate.27 In another study 
conducted in European countries, the main age for 
children who had non-protected levels against HBV 
was 9.5 years, while the main age for those who 
responded and had protected levels of antibodies 
was 5.7 years.30 

In this study, there was a difference in protection 
rate at the various annual intervals (1–5 years) since 
vaccination [Table 4]. Zhou et al. reported that 
protective levels of antibodies decrease with time;25 
however, they can remain sufficient in healthy 
individuals for at least 10 years after primary 
immunisation.31

Conclusion 
This study revealed a low coverage rate of 
HBV vaccine, and a low protective rate against 
HBV infection. A considerable proportion of 
vaccinated children should be considered for 
either revaccination or booster doses due to a non-
existent, inadequate, or low response. An effort to 
complete HBV vaccine coverage to 100% among 
the Yemeni child population is recommended, 
especially among newborns. 

c o n f l i c t o f i n t e r e s t 
The authors reported no conflict of interest.

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