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Observational study of vaccine efficacy 14 years after trial of hepatitis B vaccination in Gambian children

Hilton Whittle, deputy director ^a, Shabbar Jaffar, senior lecturer ^b, Michael Wansbrough, MSc student ^b, Maimuna Mendy, senior scientific officer ^a, Uga Dumpis, visiting research fellow, Riga University ^a, Andrew Collinson, clinical scientist ^a, Andrew Hall, professor ^b. 

^a Medical Research Council Laboratories, PO Box 273, Banjul, Gambia, ^b London School of Hygiene and Tropical Medicine, London WC1E 7HT

Correspondence to: H Whittle hwhittle{at}mrc.gm

	Abstract

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Objective: To determine the duration of protection from hepatitis B vaccine given in infancy and early childhood.
Design: Cross sectional serological study of hepatitis B virus infection in children of various ages 14 years after the start of a trial of vaccination regimens.
Setting: Two villages in the Gambia.
Participants: Children and adolescents given hepatitis B vaccine in infancy or early childhood: 232 were aged 1-5 years, 225 aged 5-9 years, 220 aged 10-14 years, and 175 aged 15-19 years.
Main outcome measures: Vaccine efficacy against infection and against chronic infection in the different age groups.
Results: Vaccine efficacy against chronic hepatitis B virus carriage was 94% (95% confidence interval 89% to 97%), which did not vary significantly between the age groups. Efficacy against infection was 80% (76% to 84%). This was significantly lower in the oldest age group (65%, 56 to 73). Of the uninfected participants in this age group, 36% had no detectable hepatitis B virus surface antibody. Time since vaccination and a low peak antibody response were the most powerful risk factors for breakthrough infection (P<0.001 in each case). Low peak antibody response was also a risk factor for chronic carriage (odds ratio 95, 19 to 466).
Conclusions: Children vaccinated in infancy are at increased risk of hepatitis B virus infection in the late teens. The risk of chronic carriage after sexual exposure needs further assessment to determine if booster vaccines are necessary.

What is already known on this topic An expert panel has declared that booster immunisations are not needed for lifelong immunity to hepatitis B The evidence for maintenance of immunity in teenagers after vaccination in infancy is slender The risk of hepatitis B virus infection is increased by sexual exposure What this study adds Teenagers vaccinated in infancy have low concentrations of antibody to hepatitis B surface antigen Even though breakthrough infections are common at this age, protection against chronic infections with hepatitis B virus may be maintained

	Introduction

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Chronic infection with hepatitis B virus is a leading cause of death from cancer in Africa; a quarter of the 60 million carriers die either of primary hepatocellular carcinoma or cirrhosis of the liver. ^{1 2} However, although hepatitis B vaccination is the simplest and most effective intervention to prevent mortality in adults both globally and in Africa,² only one country in west Africa and two in southern Africa have a continuing vaccination programme.³ We investigated vaccine efficacy against infection and protection against chronic carriage after 14 years.

	Methods

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The demographic and medical background of the villages of Keneba and Manduar, which in 1998 had populations of 1474 and 607 respectively, has been described previously.⁴ Surveys of hepatitis B virus infections in these villages took place in 1973, 1980,⁴ 1984,⁵ 1989,⁶ and 1993⁷ and from November 1998 to March 1999. At the time of the third survey in November 1984 all children under the age of 5 years who were seronegative for hepatitis B virus infection were vaccinated. These children were assigned randomly to receive plasma derived vaccine against hepatitis B virus according to one of three vaccination regimens (groups 1 to 3; see bmj.com for details). Subsequent vaccination of newborn infants has continued with four doses of various vaccines given intramuscularly. Children were also randomly assigned to one of three regimens (groups 4 to 6).

Concentrations of antibody to hepatitis B virus surface antigen (anti-HBsAg) were measured two months after vaccination (peak antibody), and this and other tests for hepatitis B virus core antibody (anti-HBc) and hepatitis B virus surface antigen (HBsAg) were carried out at each survey.⁷

	Results

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By the time of the 1998 survey 1041 young people (833 aged 0-14 years, 208 aged 15 years) had been vaccinated in the two villages. We excluded from the study 29 infants who were below 1 year of age and 23 children who had received two or fewer doses of vaccine, leaving 989 children. Of these 989, 856 gave a blood sample, 64 refused to take part, 33 had died, and 36 could not be traced; coverage ranged from 94% for children aged 1-4 years to 81% for those between 15 and 19 years.

Effect of vaccination on pattern of infection
At the start of vaccination Manduar had a much higher prevalence of hepatitis B virus infection and HBsAg carriage compared with Keneba. Between 1984 and 1998 vaccination dramatically reduced the prevalence of hepatitis B virus infection in children from 48% (302/620) to 11% (68/624) in Keneba and from 80% (246/309) to 15% (34/223) in Manduar. The corresponding changes in HBsAg carriage rates were from 13% (83/622) to 1% (6/628) in Keneba and from 35% (108/309) to 2% (4/227) in Manduar.

Vaccine efficacy by village and age
Overall, crude vaccine efficacy against HBsAg carriage was 94% (95% confidence interval 89% to 97%), which did not vary significantly between villages or by age group (table 1). Overall crude vaccine efficacy against infection was 80% (76% to 84%), which did not vary between villages but differed according to age group, being significantly lower among those aged 15 years compared with any of the other younger age categories (P<0.001). After we adjusted for age and village, the overall vaccine efficacy against carriage was 94% (89% to 97%) and against infection was 82% (78% to 85%).

Duration of response and breakthrough infections in children immunised in 1984
The participants had a median age in 1998 of 16.2 years (range 14.2-21.7 years) and had been followed up for a median of 13.8 (13.5-14.1) years. In each of the groups, which had significantly different peak antibody responses in 1985 (P<0.0001), antibody decayed in a similar and regular exponential manner with time. The proportion of uninfected participants with undetectable antibody concentrations (<10 mIU/ml) differed between the groups (P=0.001) and increased with time (P<0.0001) (see bmj.com).

The proportion of breakthrough infections and the cumulative proportion of breakthrough infections (consisting of current infections and past infections that were no longer detectable) also increased with time (P<0.0001 in both cases), but neither of these proportions differed significantly between the groups. By 1998, 64 of the 171 (37%) vaccinated participants had been infected, and of the 111 uninfected participants, 40 (36%) had undetectable concentrations of antibody.

Vaccine efficacy against infection was 49% (28% to 64%), 36% (5% to 57%), and 92% (57% to 100%) for vaccination regimen groups 1, 2, and 3, respectively (P<0.01 for comparison between group 1 or 2 and 3). In 1998 one of 54, 4 of 57, and none of 64 participants in groups 1, 2, and 3 were chronic carriers of HBsAg. Two of the chronic carriers were infected within a year of vaccination; the three others were infected five or more years later. Vaccine efficacy against chronic carriage was 91% (37% to 99%), 66% (11% to 87%), and 100% for groups 1, 2, and 3, respectively.

Breakthrough infections and chronic carriage according to peak antibody responses
The number of breakthrough infections was related to vaccination group (P=0.01) and to the peak antibody concentration (P=0.001) (see table 3 in full version on bmj.com).Those with an undetectable response (equivalent to <10 mIU/ml) had six times the chance of infection compared with those with high responses (>999 mIU/ml). More importantly, participants whose peak antibody response was <10 mIU/ml were 75 times more likely to become chronic carriers than those with responses  10 mIU/ml (P<0.0001). Seven out of the 10 chronic carriers, all of whom had a peak antibody response of less than 10 mIU/ml, were infected before the age of 5 years.

Predictors of breakthrough infections and chronic carriage
Time since vaccination and peak antibody concentrations were strongly associated with breakthrough infection in a logistic regression model; sex and village had a significant but lesser effect (table 2). Dose (three or four) was not significant, neither was route of administration.

The only factor associated with chronic carriage of hepatitis B virus was a peak response of <10 mIU/ml (8/39 (21%) versus 2/731 (0.3%) children with a higher response (odds ratio 95, 95% confidence interval 19 to 466)).

	Discussion

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Natural boosting and immunity
The role of natural boosting in maintaining immunity in highly endemic settings is not clear. In our study antibody concentrations in uninfected older teenagers stabilised over the previous four years, perhaps as a result of increased exposure by the sexual route. In a larger cohort of Gambian people vaccinated in infancy, we have noted transient rises in antibody concentrations, which may be due to transient infections and which probably boost both cellular and humoral immunity. ^{9 10} The role, if any, that sexual exposure plays in maintaining and boosting immunity remains to be defined and at present there are insufficient data to decide if a booster dose would be useful in teenagers in highly endemic areas where 15% of sexual partners may be chronic carriers of hepatitis B virus. ^{4 11} In an area of low endemicity teenagers vaccinated in infancy may lose immunity because of lack of exposure, and a booster dose may be necessary at the onset of sexual maturity. In this setting it may be more sensible to deliver the primary course of vaccine in adolescence.

Risk factors for breakthrough infection
Independent risk factors associated with breakthrough infection were sex, village, time since vaccination, and peak antibody response. Boys and young men had a higher risk, as did people living in Manduar, which before vaccination had a remarkably high rate of infection of 71% in young children over a four year period.⁵ As age and type of vaccine were directly linked in groups 4, 5, and 6 we were not able to analyse these effects separately for the whole dataset. However, in participants in groups 1, 2, and 3, who were simultaneously given different doses by different routes, time since vaccination seemed to be a major determinant. Breakthrough infections and chronic carriage were clearly and strongly related to peak antibody concentrations. Thus half of the children who failed to produce detectable concentrations of antibody became infected, most within the first five years after vaccination, and of those infected nearly half became chronic carriers. However as the numbers were small and as vaccines, dose, and routes of administration varied we were not able to assess formally which of these factors were the most important determinants of breakthrough infections resulting in chronic carriage of hepatitis B virus.

Other long term follow up studies
Few of the follow up studies of infants or young children vaccinated against hepatitis B virus have lasted longer than 10 years. ^{8 12-14} The longest was in China, where 52 of the original 477 children (11%) were followed for 15 years: half had detectable concentrations of anti-HBsAg, and vaccine efficacy was 87% for chronic carriage and 86% for infection.¹⁴ This small study with a large dropout rate formed the basis for a recent consensus statement that no hepatitis B virus booster was required for 15 years after primary vaccination.¹⁵ However as the age at first sexual intercourse may be higher in China than in Africa the findings may not be generally applicable to other groups who start sexual activity earlier.¹⁶

In conclusion, our long term study of infant hepatitis B virus vaccination in infancy in a highly endemic country showed that vaccine efficacy against infection waned with time but efficacy against chronic infection remained high over 14 years. However, the numbers involved are relatively small and a larger study of efficacy during adolescence is necessary before we conclude that a booster dose is not needed¹⁵ before the onset of sexual activity. In Africa and elsewhere, the risk of infection and of chronic carriage might be increased by the presence of other sexually transmitted infections, as is the case for HIV-1.¹⁷

	Acknowledgments

We thank Lamin Giana, Joseph Bass, and Adam Jeng for their help in the laboratory and field.

Contributors: see bmj.com

	Footnotes

Funding: Medical Research Council (UK) and Merck Sharpe & Dohme (USA).

Competing interests: None declared.

The full version of this article appears on bmj.com

	References

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(Accepted 23 April 2002)