Over 73 per cent of vaccinated patients developed measles attack before an interval of six months of vaccination, thereby indicating a low level of antibody titer against measles infection after the receipt of vaccine, acccording to a study conducted by Anjali Puri, V.K. Gupta A. Chakravarti M. Mehra from the Departments of Preventive and Social Medicine and Microbiology, Maulana Azad Medical College in 30 clusters of slums of Delhi.
Seven measles vaccinated and seven measles unvaccinated children of the age group 12-35 months were selected randomly from each cluster, thus a total of 420 children were included in the study. Children who had received measles vaccination before the age of nine months and who had an attack of measles prior to vaccination were excluded from the study.
The overall attack rate in the study children was found to be 26%. The difference in the attack rates of the vaccinated and unvaccinated children (14.3% and 37.6%, respectively) was statistically significant.
The relative risk was calculated to be 2.6 indicating that an unvaccinated child is 2.6 times more prone to develop measles as compared to a vaccinated child. In the present study, the vaccine efficacy by case- reference method was 62%.
The risk ratio of 2.6 obtained in the study is concordant with risk ratio obtained in other trials, which reported that an unvaccinated child was 2-3 times more prone to develop measles as compared to a vaccinated child.
A similar study conducted in Najafgarh zone of Delhi observed attack rates of 14.7% and 31.3% in the vaccinated and unvaccinated children, respectively. The vaccine efficacy in the same study was calculated to be 53.1%.
A multi-centric study also reported the vaccine efficacy range from 61.7% to 75.1%. In an outbreak in India in six south Indian villages in 1980, it was seen that none of 121 immunized children developed measles but 198 of unimmunized children did. The vaccine efficacy was thus calculated to be 100%.
The effectiveness of a vaccine to prevent disease effectively is termed as vaccine efficacy. It can be defined as a per cent reduction in disease incidence attributable to vaccination. Though potency is tested before distribution, there is a need to study the efficacy of vaccines under field conditions, the study authors said.
It has been realized that most of the laboratory-based trials on vaccine efficacy are carried out under controlled conditions, giving results which are unrealistic in the field conditions.
Under UIP, many different immunization centers and vaccinators are involved. The success of vaccination performed under field conditions is more realistically assessed by measuring protection against the disease by epidemiological methods such as screening, outbreak, investigations, secondary attack rates in families or clusters, coverage survey methods in endemic areas and case control studies.
Under field conditions immunization cannot be strictly supervised and many factors affect the subsequent development of protective antibodies. Vaccine efficacy obtained under field conditions, thus realistically assess the protection afforded by the vaccine against the diseases.
Speaking to Pharmabiz.com, Manish Vadva of The Serum Institute of India, which sells the M-Vac Sii measles vaccine brand, reasoned that the high rate of measles within six months of vaccination must be due to the presence of antibodies in the child at the time of receipt of the vaccine. If these are present, the level of antibodies produced are affected, he said. The other brand in the market is Rimevax(Smithkline Beecham). Haffkine Institute Director Dr.S.M. Sapatnekar said that measles is growing to be the number one disease in children. He called for stronger research efforts in improving the efficacy of measles vaccines.
Inability to show protective antibody titer may be due to waning of antibody titer or lack of initial seroconversion, the authors say. Since seroconversion is rarely documented after vaccination, therefore, it is difficult to distinguish these two types, the study said.
The lack of initial seroconversion may be due to usage of inactive vaccine at the time study group was vaccinated or due to host factors such as genetic/physiological unresponsive-ness, inappropriate age, malnutrition, inter-current infection, waning immunity levels etc, the authors added.