Letters to the Editor

Indian Pediatrics 2002; 39:787-788

HBV Carrier Rate in India

Drs. John and Abraham, in their letter have claimed that "The extensive review by INSAL, had arrived at a consensus figure of 4.7% as the national average for carrier state"(1). This claim of HBV carrier rate of 4.7% in India is based on the estimation by Thyagarajan et al(2). This estimation suffers from 3 errors: (i)Positivity rate has been confused with carrier rate - The studies used are all one time, cross-sectional studies of prevalence of HBsAg positivity in mostly blood donors. This positivity rate is quite a different indicator than carrier rate. Carrier stage in HBV-infection is persistence of infection for six months or more(3); (ii) Thyagarajan et al have included 3 studies on professional blood donors and one from dental population. It is wrong to include such high risk groups in estimating prevalence in general population; (iii) An elementary arithmetical error has been committed in calculating the average from various studies. The average prevalence of 4.7% has been calculated by simply taking average of the averages of the individual studies! Scientific way would be to calculate, the weighted average as has been done in table 2 (prevalence in pregnant women) in the next chapter of the book in which this estimation has been published.

We have used scientifically, the same data used by Thyagaraj et al. We have excluded the studies on professional blood donors and dental personnel, and have calculated the weighted average of the HBsAg positivity rate in different centres. In this process, we had to exclude studies, which have not mentioned the number of persons tested. The weighted average of the positivity rate in the remaining studies was found to be 2.64%. This weighted average broadly agrees with the data available from other studies. For example, in the same book in which Thyagrajan et al’s paper has been published, a study of HBsAg positivity rate in pregnant women coming to ANC clinics found this rate to be 2.8%(4). In our forthcoming paper, we have summarised the results of 18 studies between 1991 and 1999. This gives an average HbsAg positivity rate of 2.41%.

This single test positivity rate of 2.64% based on the data used by Thyagrajan et al, is not the point-prevalence. To find out what proportion of the HBsAg positives would actually be infected with the HB-virus, we would have to apply the corrective factor of Positive Predictive Value (PPV). Assuming the sensitivity and specificity of the HBsAg test to be 100 and 99 percent respectively, the PPV of this screenings test is 67.1% at the prevalence rate of 2%. Assuming for the observed prevalence of HBsAg positivity, the corresponding figure would thus be 2.64 × 67.1% = 1.77%. Studies, which have followed up initial HBsAg positives for six months, have found that about 75 to 80% of these positive continue to be positive and hence are carriers(5-7). Extrapolating these findings to the above estimation of HBsAg point-prevalence of 1.47% in India. HBsAg-carrier rate works out to be 1.77% × 0.80 = 1.42% (Phadke Anant, Kale Ashok. Some Critical Issues in the Epidemiology of Hepatitis - B in India. Indian Journal of Gastroenterology, 2000, vol. 19 (Suppl. 3) December, C76-C77). The study quoted by John/Abraham (Singh J. Prakash C. et al) has tested the blood twice for HBsAg, but not 6 months apart. Hence their study does not give carrier rate, but gives only the point-prevalence.

The WHO has grouped countries into three categories. Countries with Hep-B carrier rates of upto 2%, 2 to 7% and above 7% are to be grouped into low, intermediate and high prevalence zones. Universal hep-B immunization has been recommended only for prevalence of more than 2% carrier rate(8).

The IAP recommendation of universal immunization of all the infants is based on an unscientific assumption of a carrier rate of 4.7% and hence need to be revised.

References

1. John TJ, Abraham P. Heptatitis B in India: A review of disease epidemiology. Indian Pediatr 2001; 38: 1318-1322.

2. Thyagrajan SP, Jayaram S, Mohanvalli B. Prevalence of HBV in the General Population of India, in Hepatitis B in India, Ed. Sarin SK, Singhal AK, CBS Publishers & distributors, 1996, P.9.

3. Public Health and Preventive Medicine, Maxcy- Rosenau-Last, 14th Edn. Ed. Wallace Robert B, Appleton - Lange, 1998, p. 180.

4. Kant L, Arora N. Transmission of Hepatitis B virus in children - Indian scenario. In Hepatitis B in India, Ed. Sarin SK, Singhal AK, 1996; op. cit. p.9.

5. Alward WLM, Mcmohan BJ, Hall DB et al. The Long-Term Serological Course of Asymptomatic Hepatitis B Virus Carriers and the Development of Primary Hepatocellular Carcinoma. J Infect Dis 1985; 15: 605.

6. Gupta I, Sehgal R. et al. Vertical Transmission of Hepatitis B in North India. J Hyg Epidemiol Microbio Immunol, 1992; 36: 265.

7. Elavia AJ & Bankar DD. Prevalence of hepatitis B surface antigen & its subtypes in high risk group subjects & voluntary blood donors in Bombay, Indian J Med Res 1991; (A)93; 280-285.

8. Ghendon Y. WHO Strategy for the global diminution of new cases of hepatitis B. Vaccine 1990, 8: S 129-133.