Two-dose measles vaccine needed in crowded areas

NEW YORK (Reuters Health) - Evidence from a recent large measles outbreak in the Republic of the Marshall Islands (RMI) indicates that, in very crowded living conditions, routine two-dose measles vaccination schedule reaching most of the inhabitants is required to provide measles "herd immunity."

Herd immunity occurs when a large segment of the population is vaccinated against a particular microbe, which, in turn, makes the odds of infection among unvaccinated individuals low.

The RMI instituted single-dose measles vaccination in 1982, and switched to a two-dose schedule in 1998. Between 1989 and 2002, the South Pacific nation did not report a single case of measles, but in 2003 an outbreak resulted in more than 800 measles cases, 100 hospitalizations, and three deaths. Infants below the age of routine vaccination made up 23 percent of the cases.

Only after more than 35,000 of the 51,000 population were vaccinated did the outbreak end.

Analysis of measles attack rates by Dr. Mona Marin, from the US Centers for Disease Control and Prevention in Atlanta, Georgia, and colleagues showed that vaccine efficacy was 92 percent for one measles-mumps-rubella (MMR) vaccination, and 95 percent for two doses, thus ruling out a diminished vaccine efficacy as a cause of the outbreak.

They note in their report in the journal Clinical Infectious Diseases that vaccine coverage of a single dose was 93 percent among 2-year-old children in 1998 and 80 percent in 2001. However, second-dose coverage at both time points was only 40 percent.

Another factor related to the size of the outbreak is that the RMI is densely populated, with a convenience sample of households showing a median household size of 10.5 persons, and median number of persons per room of 5.5.

"Policy makers need to consider whether aggressive implementation and maintenance of high routine 2-dose measles vaccination coverage may be needed to avert outbreaks in similar communities elsewhere and to protect infants below the age of vaccination," Marin and colleagues maintain.

"The available information suggests that (measles) eradication is scientifically feasible," Dr. Walter A. Orenstein, from Emory University in Atlanta, Georgia, and colleagues write in a related commentary.

They recommend mass campaigns every 3 to 5 years, regardless of prior vaccination status, to all children at least 1 year old born since the prior campaign.

SOURCE: Clinical Infectious Diseases, February 1, 2006.