Indian J Pcdiatr 1992; 59 : 321-323
Epidemiological Evaluation of Oral Polio Vaccine Efficacy in Delhi Jagvir Singh, Kaushal Kumar, D. Bora, Urea Chawla, N.C. Bilochi, R.S. Sharma, M.L. Kapur, Suresh Kumar, B.K. Aggarwal, J.K. Dhaon* and K.K. Dutta**
National hlstitute of Communicable Diseases, New Delhi, *State Health Institute, K.G. Medical College, Lucknow and **Minsitry of Health, Sultanate of Oman, Muscat Forty s e v e n c a s e s of poliomyelitis and 94 controls were studied for immunization status. Unmatched analysis with one control per case and two controls per case was done to find out the ratio of the odds of immunization in diseased individuals as compared with the nondiseased (odds ratio). This ratio (OR) was used further to calculate oral polio vaccine efficacy. OPV efficacy was found to be 93% with 95% confidence limits of 75-98%.
Keywords : Oral polio vaccine, Vaccine efficacy. The 42nd World Health Assembly has issued the challenge of achieving global polio eradication by the year 2000. However, poliomyelitis is still the major cause of lameness of children of age 5-9 years in India. Based on a special combined survey for poliomyelitis and neonatal tetanus in 1981, it was estimated that 140,000 to 170,000 children in the country get poliomyelitis every year? As a minimum estimate, 60,000 children per year are still stricken with paralytic poliomyelitis.2 Oral polio vaccine was included in the nationwide immunization programme at the end of 1979. In 1989, the reported coverage levels were estimated to be around 75%? The high polio incidence in spite of Reprint requests : Dr. Jagvir Singh, Assistant Director, Epidemiology Division, National Institute of Communicable Diseases, 22 Shamnath Marg, Delhi-ll0 054.
75% immunization coverage with 3 doses of OPV may be due to vaccine failure. Therefore, periodic evaluation of field vaccine efficacy is a must for programme managers. Various approaches are available for estimating vaccine efficacy? Information on the immunization status of poliomyelitis cases and immunization coverage has been utilized to estimate vaccine efficacy of OPV in greater Bombay5 which exceeded 90%. However, this method is difficult to carry out since it requires data on immunization coverage and separate analysis of data on poliomyelitis cases by age. Other methods of estimating vaccine efficacy include case control studies? This method is eminently suited to our country, because it not only estimates vaccine efficacy quickly and without much efforts and money, but also elucidates non-immunization as a risk factor of paralytic poliomyelitis. Thus, the present study is an attempt to
321
322
T I l E INDIAN J O U R N A L O F P E D I A T R I C S
Vol. 59. No. 3
evaluate the efficacy of OPV by case control study,
(odds ratio). This ratio (OR) was used further to calculate vaccine efficacy.
MATERIAL AND METHODS
RESULTS
Kalawati Saran Children Hospital is visited by a majority of polio cases in Delhi. Forty seven such children who developed disease in the year 1990, were traced in the community from the addresses available in hospital records. Two controls were selected for each case who were of same sex, residing in the neighbourhood and of the same socio-economic environment. Matching of the age of the cases with the controls was done as below :
Forty seven cases and 94 controls were studied. When analysed unmatched, 10 cases and 73 controls were vaccinated (Table 1). The OR was found to bc 0.08. Vaccine efficacy was found to be 92%. Matched analysis with one control per case (Table 2) gave the OR as 0.07 for first or second control. Vaccine efficacy was calculated as 93%. In triplet analysis (Table 3), the O R was 0.07 (upper limit = 0.25; lower limit = 0.02 with 95% confidence). Vaccine efficacy was found :o be 93% (upper limit
Age group
TAaLE1. Unmatched Analysis of Poliomyelitis Cases and Controls
Case
Control
0 - < 1 year
__. 1 month
1- < 2 years
_.+ 2 months
2 - < 3 years
-+ 3 months
3 - < 4 ycars
_.+ 4 months
4 - < 5 years
-+ 5 months
Immunization status of all children was checked and verified from immunization cards. Whercever cards were not available, it was verified from the records of local heahh workers. The child having less than 3 doses of OPV (at least 4 weeks apart) before the onset of disease (in the case) was considered non-immunized. Interval between the last dose of OPV and onset of the disease was taken to be 2 weeks for vaccine faiturc. Unmatched analysis, matched analysis with one control per case and two controls per case was done '~ to find out the ratio of the odds of immunization in diseased individuals as compared with the nondiseased
Immunization
Case
Control
Total
Immunized
l0
73
83
Non-Immunized
37
21
58
Total
47
94
141
Odds ratio =0.08, Vaccine effica~=92% TABLE 2. Poliomyelitis Cases and Controls Matched Analysis With One Control per Case Control 1
+
Control 2
+ *
,*,r
+
8
2
8
2
30
7
27
t0
Case -
Odds ratio =0.07 Vaccine efficacy=93%
Odds ratio =0.07 Vaccine efficacy=93%
* Child immunized, ** Child non-immunized
SINGII [',"F A L : E V A I , U A T I O N OF OILAL POLIO VACCINE
"rABLE3. Efficacy of Oral Polio Vaccine by Matched Analysis With Two Controls per Case of Poliomyelitis %=8 (-~++) *n4=23(-++) Oddsratio=0.07 n~=0 (++-) n5=7 (-+-) X2=36.84; d.f=l n2=0 (+-+)
n6=4
n3=2 ( + - )
n7=3
(-- +) Vaccineeffi cacy=93% (---)
* 9 = Case or control immunized; .- = nort/mmunized; First + or - pertains to case, while 2nd and 3rd pertain to first and second controls respectively. 98%; lower limit = 75% with 95% confidence). DISCUSSION Vaccine potency testing and serological studies can be used to determine vaccine efficacy? However, these procedures are expensive and require laboratory support. Vaccine efficacy is more economically assessed by measuring protection against disease by cpidemiologic means which do not require laboratory support. C~se control studies are practical in case of poliomyelitis, as very large samples are required if cross-sectional studies are done. Cohort studies would be unethical. In India, personal immunization records are generally not available. However, intensive efforts can be used to determine vaccination status of a limited number of cases and non cases, instead of concentrating on the whole population. By knowing the vaccine history of cases and of non-cases, the OR and hence vaccine efficacy can be estimated. Vaccine efficacy was found to be 93% by triplet analysis (one case - two controls).
323
With 95% confidence, the upper limit was 98% and lower limit 75%. High vaccine efficacy indicates that poliomyelitis in Delhi may be due to incomplete vaccine coverage rathar than by vaccine failure. Non-immunization was found to be a highly significant risk factor (P < 0.001). ACKNOWLEDGEMENTS
The authors are extremely grateful to Dr. T. Verghese, Director, National Institute of Communicable Diseases (NICD), Delhi, for providing the opportunity to undertake the study. The authors are also thankful to Dr. G. Stroh, Consultant,CDC, Atlanta, USA, for useful suggestions in preparation of this report. Thanks are also due to the staff of NICD for their continued support and assistance. REFERE,NCES
1. Basu RN, Sokhcy J. Special combined survey for poliomyelitis and neonatal tetanus as supplement to routine surveillance system in India. J Comm Dis 1984; 16 : 148153. 2. Rohde JE. A strategw for strengthening primary health care. b~dian J Comm ,~fed 1990; 15 : 185-187. 3. Sokhey J. The immunization program in India. hzdiar J Comm Med 1~)0; 15 : 163-172. 4. Orertstein WA, Bernier RH, Dondero TJ et al. Field evaluation of vaccine efficacy. WItO Document 1984; EPI/GEN/84/10. Rev. 2. 5. Kim-Farley R J, Dave KH, Sokhey J, Mandke VB. Poliomyelitis surveillance and vaccine efficacy in Bombay, 1982-1987. Bull I~7f0 1989; 67 : 663-667. 6. Schlesselman JJ. Case Control ~tudies. New York : Oxford University Press, 1982 : 174216.
Epidemiological Evaluation of Oral Polio Vaccine Efficacy in Delhi Jagvir Singh, Kaushal Kumar, D. Bora, Urea Chawla, N.C. Bilochi, R.S. Sharma, M.L. Kapur, Suresh Kumar, B.K. Aggarwal, J.K. Dhaon* and K.K. Dutta**
National hlstitute of Communicable Diseases, New Delhi, *State Health Institute, K.G. Medical College, Lucknow and **Minsitry of Health, Sultanate of Oman, Muscat Forty s e v e n c a s e s of poliomyelitis and 94 controls were studied for immunization status. Unmatched analysis with one control per case and two controls per case was done to find out the ratio of the odds of immunization in diseased individuals as compared with the nondiseased (odds ratio). This ratio (OR) was used further to calculate oral polio vaccine efficacy. OPV efficacy was found to be 93% with 95% confidence limits of 75-98%.
Keywords : Oral polio vaccine, Vaccine efficacy. The 42nd World Health Assembly has issued the challenge of achieving global polio eradication by the year 2000. However, poliomyelitis is still the major cause of lameness of children of age 5-9 years in India. Based on a special combined survey for poliomyelitis and neonatal tetanus in 1981, it was estimated that 140,000 to 170,000 children in the country get poliomyelitis every year? As a minimum estimate, 60,000 children per year are still stricken with paralytic poliomyelitis.2 Oral polio vaccine was included in the nationwide immunization programme at the end of 1979. In 1989, the reported coverage levels were estimated to be around 75%? The high polio incidence in spite of Reprint requests : Dr. Jagvir Singh, Assistant Director, Epidemiology Division, National Institute of Communicable Diseases, 22 Shamnath Marg, Delhi-ll0 054.
75% immunization coverage with 3 doses of OPV may be due to vaccine failure. Therefore, periodic evaluation of field vaccine efficacy is a must for programme managers. Various approaches are available for estimating vaccine efficacy? Information on the immunization status of poliomyelitis cases and immunization coverage has been utilized to estimate vaccine efficacy of OPV in greater Bombay5 which exceeded 90%. However, this method is difficult to carry out since it requires data on immunization coverage and separate analysis of data on poliomyelitis cases by age. Other methods of estimating vaccine efficacy include case control studies? This method is eminently suited to our country, because it not only estimates vaccine efficacy quickly and without much efforts and money, but also elucidates non-immunization as a risk factor of paralytic poliomyelitis. Thus, the present study is an attempt to
321
322
T I l E INDIAN J O U R N A L O F P E D I A T R I C S
Vol. 59. No. 3
evaluate the efficacy of OPV by case control study,
(odds ratio). This ratio (OR) was used further to calculate vaccine efficacy.
MATERIAL AND METHODS
RESULTS
Kalawati Saran Children Hospital is visited by a majority of polio cases in Delhi. Forty seven such children who developed disease in the year 1990, were traced in the community from the addresses available in hospital records. Two controls were selected for each case who were of same sex, residing in the neighbourhood and of the same socio-economic environment. Matching of the age of the cases with the controls was done as below :
Forty seven cases and 94 controls were studied. When analysed unmatched, 10 cases and 73 controls were vaccinated (Table 1). The OR was found to bc 0.08. Vaccine efficacy was found to be 92%. Matched analysis with one control per case (Table 2) gave the OR as 0.07 for first or second control. Vaccine efficacy was calculated as 93%. In triplet analysis (Table 3), the O R was 0.07 (upper limit = 0.25; lower limit = 0.02 with 95% confidence). Vaccine efficacy was found :o be 93% (upper limit
Age group
TAaLE1. Unmatched Analysis of Poliomyelitis Cases and Controls
Case
Control
0 - < 1 year
__. 1 month
1- < 2 years
_.+ 2 months
2 - < 3 years
-+ 3 months
3 - < 4 ycars
_.+ 4 months
4 - < 5 years
-+ 5 months
Immunization status of all children was checked and verified from immunization cards. Whercever cards were not available, it was verified from the records of local heahh workers. The child having less than 3 doses of OPV (at least 4 weeks apart) before the onset of disease (in the case) was considered non-immunized. Interval between the last dose of OPV and onset of the disease was taken to be 2 weeks for vaccine faiturc. Unmatched analysis, matched analysis with one control per case and two controls per case was done '~ to find out the ratio of the odds of immunization in diseased individuals as compared with the nondiseased
Immunization
Case
Control
Total
Immunized
l0
73
83
Non-Immunized
37
21
58
Total
47
94
141
Odds ratio =0.08, Vaccine effica~=92% TABLE 2. Poliomyelitis Cases and Controls Matched Analysis With One Control per Case Control 1
+
Control 2
+ *
,*,r
+
8
2
8
2
30
7
27
t0
Case -
Odds ratio =0.07 Vaccine efficacy=93%
Odds ratio =0.07 Vaccine efficacy=93%
* Child immunized, ** Child non-immunized
SINGII [',"F A L : E V A I , U A T I O N OF OILAL POLIO VACCINE
"rABLE3. Efficacy of Oral Polio Vaccine by Matched Analysis With Two Controls per Case of Poliomyelitis %=8 (-~++) *n4=23(-++) Oddsratio=0.07 n~=0 (++-) n5=7 (-+-) X2=36.84; d.f=l n2=0 (+-+)
n6=4
n3=2 ( + - )
n7=3
(-- +) Vaccineeffi cacy=93% (---)
* 9 = Case or control immunized; .- = nort/mmunized; First + or - pertains to case, while 2nd and 3rd pertain to first and second controls respectively. 98%; lower limit = 75% with 95% confidence). DISCUSSION Vaccine potency testing and serological studies can be used to determine vaccine efficacy? However, these procedures are expensive and require laboratory support. Vaccine efficacy is more economically assessed by measuring protection against disease by cpidemiologic means which do not require laboratory support. C~se control studies are practical in case of poliomyelitis, as very large samples are required if cross-sectional studies are done. Cohort studies would be unethical. In India, personal immunization records are generally not available. However, intensive efforts can be used to determine vaccination status of a limited number of cases and non cases, instead of concentrating on the whole population. By knowing the vaccine history of cases and of non-cases, the OR and hence vaccine efficacy can be estimated. Vaccine efficacy was found to be 93% by triplet analysis (one case - two controls).
323
With 95% confidence, the upper limit was 98% and lower limit 75%. High vaccine efficacy indicates that poliomyelitis in Delhi may be due to incomplete vaccine coverage rathar than by vaccine failure. Non-immunization was found to be a highly significant risk factor (P < 0.001). ACKNOWLEDGEMENTS
The authors are extremely grateful to Dr. T. Verghese, Director, National Institute of Communicable Diseases (NICD), Delhi, for providing the opportunity to undertake the study. The authors are also thankful to Dr. G. Stroh, Consultant,CDC, Atlanta, USA, for useful suggestions in preparation of this report. Thanks are also due to the staff of NICD for their continued support and assistance. REFERE,NCES
1. Basu RN, Sokhcy J. Special combined survey for poliomyelitis and neonatal tetanus as supplement to routine surveillance system in India. J Comm Dis 1984; 16 : 148153. 2. Rohde JE. A strategw for strengthening primary health care. b~dian J Comm ,~fed 1990; 15 : 185-187. 3. Sokhey J. The immunization program in India. hzdiar J Comm Med 1~)0; 15 : 163-172. 4. Orertstein WA, Bernier RH, Dondero TJ et al. Field evaluation of vaccine efficacy. WItO Document 1984; EPI/GEN/84/10. Rev. 2. 5. Kim-Farley R J, Dave KH, Sokhey J, Mandke VB. Poliomyelitis surveillance and vaccine efficacy in Bombay, 1982-1987. Bull I~7f0 1989; 67 : 663-667. 6. Schlesselman JJ. Case Control ~tudies. New York : Oxford University Press, 1982 : 174216.
