ORIGINAL

Niger J Paed 2013; 40 (3): 254 –258

Ladapo TA

Antibody response to routine

measles vaccination among a

population of Nigerian children and

evaluation of vaccine potency

Egri-Okwaji MTC

Njokanma OF

Omilabu SA

DOI:http://dx.doi.org/10.4314/njp.v40i3,10

Accepted: 8th February 2013

Abstract Background: Despite a

global decline in mortality and

morbidity from measles in the last

decade, outbreaks continue to

occur in some parts of the world

including Nigeria.

Objective: To determine antibody

response to routine measles vacci-

nation in Nigerian children and

evaluate vaccine potency.

Results: Twenty seven(11.5%) had

pre-vaccination antibodies.

(

)

Ladapo TA,

Seroconversion rate among the

195 who returned for post-

vaccination sampling was 69.2%:

It was however 74.2% in children

with no pre-vaccination antibodies

compared to 17.6% in those with

antibodies. (p<0.05). Only six

(50%) of the measles vaccine vials

were potent. Seroconversion rate

among subjects vaccinated from

potent vials was 74.3% compared

with 42.9% in those vaccinated

from non-potent vials (p = 0.006).

Conclusion: Seroconversion to

measles vaccination in our envi-

ronment is sub-optimal, partly

attributable to low vaccine po-

tency. Improvement of vaccine

handling processes and booster

doses of the vaccine are recom-

mended.

Department of Paediatrics

Lagos University Teaching Hospital,

Idi-Araba, Lagos, Nigeria.

Email: drteeladapo@yahoo.com

Tel : +2348024245061

Egri-Okwaji MTC, Njokanma OF

Department of Paediatrics,

College of Medicine, Lagos / Lagos

University Teaching Hospital Lagos

State, Nigeria.

Methods: A prospective study of

2

34 children selected from 3

health centres in an urban area of

Lagos, Nigeria. Blood was ob-

tained before and 8-12 weeks af-

ter routine vaccination with Ed-

monston-Zagreb strain of measles

vaccine. Antibodies were detected

using the measles antibody neu-

tralization test. Reconstituted vac-

cines samples were analysed for

potency on monolayers of Vero

slam cells in 96-well tissue cul-

ture plates.

Omilabu SA

Department of Medical Microbiology

and Parasitology

Lagos University Teaching Hospital,

Idi-Araba, Lagos, Nigeria.

Introduction

recommends a second dose in childhood either rou-

tinely or through supplementary vaccination activities.

1

The burden of measles, a highly communicable, vaccine

Between 2006 and 2007, over 56 million Nigerian chil-

dren aged 9 months to 15 years were vaccinated through

massive nation₆ wide measles supplementary immuniza-

tion activities. This contributed to the >70% decline in

1-

p₂reventable, disease is highest in developing countries.

In 2008, there were over 270,000 reported cases with

about 197,000 deaths globally, 95% of them in the de-

2

7

veloping world. Although global measles deaths fell by

global measles mortality during that period. In spite of

7

efforts at vaccination, there have been continued reports

of outbreaks in some developing countries, including

Nigeria. Recently, however, there appears to be a resur-

gence in measles outbreaks in developed countries like

the United States of America where measles was previ-

ously reported to be practically eradicated. This has

largely been attributed to imported cases of measles as

well as waning vaccine immunity in previously vacci-

4% between 2000 and 2007 largely due to intensified

this however, outbreaks continue to occur in the coun-

try. The Paediatric Association of Nigeria (PAN) as

2

part of it’s commitment to reducing the burden of vac-

cine preventable diseases in Nigerian children, therefore

recently reco₈ mmended a second dose of measles vaccine

in children. Sero-epidemiological studies are essential

for the monitoring and evaluation of the effectiveness of

vaccination programmes. This study is designed to

assess seroconversion rates after the nine month vacci-

nation among a group of infants and evaluate vaccine

potency in Lagos, Nigeria.

2

3

nated children .

In Nigeria, measles vaccine is routinely administered at

the age of_t_hnine months. Clinically apparent measles be-

fore the 9 month vaccination and in vaccinated chil-

4

-5

dren, however suggest low passive antibody positiv-

ity levels and failure of seroconversion respectively. The

World Health Organization (WHO) therefore

Materials and methods

It was a prospective study conducted between January

2

56

between children with and those without pre-vaccination

antibodies was found to be statistically significant. p=

<

0.05 (Fisher’s exact test)

Effect of vaccine potency on seroconversion

Twelve vials were analysed for potency. Six (50%) had

potency levels_- above the minimum WHO required stan-

3

dard of log10 TCID50. (Table 4). The vaccine potency

rates were 60%, 25% and 66.7% from centers A, B and

C respectively. Ninety-four subjects were vaccinated

from these vials. Forty-three received potent vaccines of

which 35 returned for the post-vaccination with serocon-

version in 26. Non-potent vaccines were administered to

5

1 of which 42 returned for the post-vaccination sam-

pling with seroconversion in 18. The seroconversion

rates in the two groups were thus 74.3% and 42.9% re-

o

spectively. x =7.70, df1= 1, p=0.006. The duration of

administration of the vials ranged from 45minutes to

1

18minutes but this was found not to be related to the

vaccine titre. (r=0.23, p=0.48).

Table 4: Potency and duration of administration of measles

vaccines at the three centres

Centre Vaccine

Duration

minutes

(CCID50)per dose

A

4.5

4.0

2.0

118

75

62

90

A

B

C

1.0

3.5

2.0

1.5

3.5

3.0

1.0

65

45

72

55

87

85

105

75

Table 3: Distribution of seroconversion following measles

vaccination by centre.

Seroconversion

Centre

No

Yes

Total

N ( % )

N (%)

A

All

No PVA

B

19 (22.1) 67 (77.9)

13 (16.3) 67 (83.8)

86 (100)

80 (100)

All

No PVA 11 (36.8) 26 (70.2)

C

17 (37.8) 28 (62.2)

45 (100)

37 (100)

Discussion

The WHO recommendation to administer measles at the

age of nine months is predicated upon the need to main-

tain the delicate balance between the need for early pro-

tection of children and the limitation of possible neu-

tralization of the live vaccine by transplacentally ac-

quired antibodies. Expected seroconversion rate at this

All

No PVA

Total

All

No PVA

24 (37.5) 40 (62.5)

22 (36.1) 39 (63.9)

64 (100)

61 (100)

60 (30.8) 135 (69.2) 195 (100)

46 (25.8) 132 (74.2) 178 (100)

o

PVA= Pre-vaccination antibodies. All: x x = 4.70, df2,

p= 0.095.

age under optimal conditions, which

is not always

achievable is about 85% hence the WHO recommenda-

tion of a second dose of the v₁accine to provide

immunity in non-responders.

Effect of pre-vaccination antibodies on seroconversion

The overall seroconversion rate of 69.2% obtained in

this study is higher than most previous₁₂r_-e₁₄ports from this

When analysis was restricted to subjects with no

pre-vaccination antibodies, there was a rise in serocon-

version rates in all the centres while overall rate rose

from 69.2% to 74.2%. (Table 3).Of the 27 subjects who

had pre-vaccination antibodies, 17 returned for the post-

vaccination sampling out of which three (17.6%) sero-

converted. Five had a drop in titre, four no longer had

detectable antibodies while the titre remained the same

in the remaining five. The difference in seroconversion

country which range from 24% - 69%.

Higher rates

have been reported from similar developin₁g₆ countries

1

5

such as Turkey(77.6%) and India (>90%),

while a

rate o₇f 68.8% similar to ours was reported from thai-

1

land We also observed variable degrees of seroconver-

sion among study subjects implying variable levels of

protection. Bearing in mind that antibody titres wane

2

55

and April 2007 at three health centres in an urban area of

Lagos State, Nigeria. One centre(A) was chosen because

it also serves as the Local Government Area(LGA) vac-

cine store from where vaccines are dispensed to the

other centres. The quality of vaccines there would there-

fore represent the best offered by the LGA. The others(B

and C) were chosen by simple random sampling from a

list of all the six health centres managed by the LGA.

Ethical approval was obtained from the research and

ethics committee of the Lagos University Teaching Hos-

pital (LUTH) and informed consent from parents/

caregivers of study subjects. Calculated sample size was

virus titration was taken as the highest dilution with visi-

bly stained cells. Plates were read by two of the authors

and there was concordance in the judgments of both

observers in 421 (98.1%) of 429 readings.

Seroconversion was defined as either a change from

negative to positive or a four-fold rise in antibody titre

following vaccination. Vaccine potency was determined

according to WHO guidelines on monolayers of Vero

slam cells in 96-well tissue culture plates. Serial dilu-

tions of the vaccine were incubated with the cells at

0

36 C for 7-9 days. Virus end-point titres were calculated

in CCID₁5₀0_-₁₁per vial using the method of Reed and

1

95. An additional 39(20%) subjects were recruited be-

Muench.

Analysis was conducted in the virology

cause of anticipated attrition bringing total sample size

to 234.

laboratory of the LUTH under the supervision of a vi-

rologist trained in WHO assays.

Data Collection

Statistical Analysis

Children aged 9-12 months who were apparently healthy

were consecutively enrolled. Exclusion criteria were

parent/caretaker refusal, acute illness and a past history

consistent with ₁measles as defined by the centers for

disease control. Their names, ages, sex, weights,

lengths as well as telephone numbers and contact ad-

dresses of caregivers were obtained. About 3mls of

blood was obtained via a peripheral vein from each child

under aseptic conditions followed by a subcutaneous

injection of 0.5mls of the Edmonston-Zagreb strain mea-

sles vaccine. The vaccines were both reconstituted and

administered by National Programme on Immunization

trained nurses. All were from the same batch and within

the expiry date of the manufacturer. Subjects were then

given an 8-week appointment for repeat blood sampling

as above. Defaulters were contacted through telephone

calls or home visits and their post- vaccination visits

rescheduled within a two-week frame. The last doses of

vaccine in alternate reconstituted vaccine vials were

Data was analysed using version 16 of the Statistical

Programme for Social Sciences (SPSS) and Microsoft

Excel. Measures of central tendency such as mean and

standard deviation were generated for continuous vari-

ables like weight and height. Chi-square tests was used

to test statistical significance between discrete data.

Probability(p) value <0.05 considered statistically

significant.

Results

Pre-vaccination blood samples were obtained from 234

subjects. Their ages ranged from 8.8 to 12.1 months

with a mean of 9.5 ± 0.56 months. Mean weight and

length were 8.5 (±1.0) kg and 73.0(±2.7) cm respec-

tively with 231(99%) having weight for age Z scores

within two standard deviations of the mean. One hun-

dred and ninety-five returned for the post-vaccination

sampling resulting in 16.6% attrition rate. Of the rest,

one died following a diarrhoeal illness, six declined the

second visit, and five had relocated out of reach while

0

transported in an insulated cold box at -4 Cto the labo-

0

ratory and stored in a freezer at -80 C till time of analy-

sis. The time interval between reconstitution and deliv-

ery of the last dose in each vial was noted.

2

7 could not be contacted.

Laboratory analysis

Laboratory results

Measles neutralizing antibodies were₉d_-₁e₀tected using the

measles antibody neutralization test

which is based

The distribution of pre-vaccination antibody titres by

centre is shown in Table 1. Thirteen children (5%) had

neutralizing antibodies at the lowest dilution of 1:40,

fourteen (5.9%) between 1:80 and 1:2560 while 207

on the ability of the measles antibodies to measure virus

infectivity. Serial two-fold dilutions of each serum were

prepared starting from 1/40 to 1/2560 on 96-well tissue

culture plates. 0.1ml of measles virus stock in Vero slam

cells containing 100 TCID50 was_o_Cthen added to each

serum diluent and incubated at 37 for an hour. This

was further incubated with 0.1ml of Vero slam cells in a

(

88.5%) had no detectable antibodies. Following vacci-

nation, 151(77.4%) now had titres at dilutions ranging

from 1:40 to 1:2560 while 44 (22.6%) had no detectable

antibodies at a dilution of 1:40. (Table 2) Seroconver-

sion rates are as shown in Table 3. The overall serocon-

version rate was 69.2% but differed according to centre

though this did not reach statistical significance.

2

C0 rich environment for 7 days. Infectivity of the mea-

sles virus on tissue cells was visualised after staining

with crystal violet. The last column on each plate con-

tained the measles virus stock incubated with Vero slam

cells alone and served as controls. The end-point of the

(

p=0.095)

2

57

1

8

with time, children with lower titres may therefore be

at risk of losing their protective immunity. Seroconver-

sion increases with age at vaccination, corresponding to

reduced levels of maternal antibodies, the highest rates

in some children who supposedly received low potency

vaccines. It is possible that they were vaccinated at the

start of activities or the differences may be based on

individual immunogenicity but this may constitute room

for further research.

1

5

being in children vaccinated after the age of 12months.

The role of pre-vaccination antibodies in reducing sero-

conversio₃n as we observed has been previously docu-

1

mented. This underscores the need for a booster dose

of the vaccine especially in a measles endemic environ-

ment like ours where early exposure to natural infection

may have accounted for the presence of pre-vaccination

antibodies in some subjects.

Conclusion

The low seroconversion to measles vaccination obtained

in this study precludes the achievement of a herd immu-

nity of 90% required for measles control. There is thus a

build-up of susceptibles in the population with contin-

Though still sub-optimal, the 50% vaccine potency rate

in this study represents a significant im₁₃p_-₁r₄ovement of that

1

ued risks of outbreaks. As global elimination of measles

from previous reports of 11%- 18.2%

in the country.

will be based on successful elimination in all countries,

this may continue to compromise present efforts di-

rected at measles control. Improvement of the mainte-

nance of the cold-chain system of vaccine delivery and a

second opportunity for measles vaccine are recom-

mended. Seroepidemiological and seroconversion stud-

ies which help to highlight the features of the changing

epidemiology of the disease are a pre-requisite for effec-

tive control and serve as a guide for review of immuni-

zation policies.

This may be a reflection of better vaccine-handling prac-

tices, especially maintenance₉of the cold-chain advo-

1

cated for by previous workers. This study was however

conducted in an urban area with relatively more stable

electricity supply and findings may be differ in rural

settings where this is not the case. Reconstituted measles

vaccines should maintain potency if kept on a f₂o₀am pad

o

placed in an iced pack at 4 C for 24 hours. It was

however obvious during the field study that this tem-

perature was not always maintained. Thus, some chil-

dren vaccinated towards the end of activities may have

received vaccines whose potency had deteriorated dur-

ing the process.

Author’s Contributions

TAL, EMTC, FON, SAO: Conceptualization of the

study /critical editing of final draft.

TAL: Data collection.

TAL, EMTC, FON: Data analysis, study write-up

TAL and SAO: Laboratory analysis.

Incidentally, the duration of exposure was not directly

related to vaccine potency implying the role of other

factors such as potency of the vaccine prior to reconsti-

tution which is a reflection of the quality of storage and

transportation. Investigation of these factors was be-

yond the scope of this study but supporting evidence can

be obt₃ained from the study of Omilabu

Conflict of interest: None

Funding: None

1

et al who reported that measles vaccines in the Na-

tional Central Cold Store maintained potency while only

1

6.7% of them were potent when traced to the points of

service. It is therefore pertinent for the various agencies

involved in vaccine handling to address the issues relat-

ing to poor transportation and storage. While immune

response was mostly directly related to virus titres, an

important observation was that seroconversion occurred

Acknowledgements

The authors are grateful to Mr Seun Ilori for assistance

with the laboratory analysis.

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