Additional Evidence for the Safety of MMR and DTP Vaccines

Published:

In a large case-control study, researchers found no increase in risk for encephalitis or encephalopathy from MMR or DTP vaccination.

The results of this study are published in the September edition of the Journal of Pediatric Infectious Diseases (Ray P et al. Encephalopathy after whole-cell pertussis or measles vaccination: Lack of evidence for a causal association in a retrospective case-control study. Pediatr Infect Dis J 2006 Sep; 25:768-73.). These results lend further evidence to support the safety of these vaccines.

Summary
Whole-cell pertussis (wP) and measles vaccines are effective in preventing whooping cough and measles respectively. However, in the past there have been concerns expressed about a suspected increase in the risk of encephalopathy or encephalitis following vaccination with these vaccines. In many countries this led to a decline in these vaccination rates and subsequent outbreaks of the pertussis or measles were reported. Many developed countries switched to using an acellular pertussis vaccine (aP) which is less reactogenic and was perceived to be safer. DTP is still widely used in developing areas.

Previous studies have sought to assess whether there was an association between these vaccines and the development of encephalopathies. Many studies produced indeterminate results or failed to prove an association. Some experts who evaluated the data felt that because these neurologic outcomes are rare, the studies lacked sufficient statistical power to identify associated risk. This new study has addressed this issue by looking at a paediatric population of more than 2 million children, giving it sufficient statistical power. The results of this research conclude that there was no increased risk of encephalopathy among recipients of whole-cell pertussis or measles vaccines.

How the study was done
The researchers carried out a retrospective case-control study among children registered with four medical insurance companies in the western part of the United States. The researchers looked at hospital records of children aged 0-6 years for a 15-year period (from January 1, 1981, through December 31, 1995). They identified all children between 0-6 years of age who had been hospitalised with encephalopathy or related conditions during this time. The cause of the encephalopathy was categorised as known, unknown or suspected but unconfirmed. Up to 3 controls were matched to each case. Conditional logistic regression (specialised statistical analysis method) was used to analyse the relative risk of encephalopathy after vaccination with DTP or MMR vaccines in the 90 days before disease onset as defined by chart review compared with an equivalent period among controls.

Results
Among a population of more than 2 million children, the researchers identified 452 cases of encephalopathy. Cases were no more likely than controls to have received either vaccine < 90 days before disease onset. No distinct pattern of symptoms was seen in the children who developed encephalitis/encephalopathy < 14 days after DTP vaccination, or < 30 days after MMR vaccination.

Conclusions
In this study of more than 2 million children, DTP and MMR vaccines were not associated with an increased risk of encephalopathy after vaccination. This study provides strong evidence for the safety of MMR and DTP vaccines.