POST 00581E : SECOND DOSE OF MEASLES

POST 00581E : SECOND DOSE OF MEASLES Follow-up on Post 00576E 9 July 2003 ____________________________________________ Four contributions were received in response to Anil Varshney's request for advice. The first (1-) is from Adelaide Shearley (mailto:[log in to unmask]) from Namibia. The next contribution (2-) comes from Dr. Osman Mansoor (mailto:[log in to unmask]) from WHO/WPRO. Tidiane Sidibe (mailto:[log in to unmask]) then shares his opinion (3-) based on African experience. Finally, Dr. Bradley Hersh (mailto:[log in to unmask]) from WHO also contributes substantially (4-), with bibliographical references. ____________________________________________ 1- This is a good discussion issue. However, I would like to know at what age the children in this country receive their measles vaccine routinely and also the efficacy of the vaccine they are currently using. Adelaide Shearley EPI Focal Point Namibia _________________________________________ 2- Without a second dose a large measles epidemic will occur in this country. The time can be predicted with some accuracy based on past coverage and cases data, and the age at which measles vaccine is given. If you send me that data, I can try and predict for you. Not only will the health sector be unprepared for the large number of cases after not having seen measles for many years, there will also be many cases in young adults who will have much higher morbidity and mortality than young children. The epidemic is inevitable because of the build up of susceptibles (from vaccine failure and failure to vaccinate) to the threshold needed for epidemic spread of measles (generally believed to be about one or two birth cohorts or so - but a lot of this depends on how you calculate etc. - and subject of some controversy). This is the reason for the "second opportunity" that is now WHO policy. In countries that achieve a high coverage with first dose, the second opportunity can work well as a scheduled second dose. Regular campaigns to deliver also has some advantages, and there may important "holes" in immunity in older children in the country you refer to that would be best "filled" through a wide age-range campaign. To prevent epidemics it is necessary to maintain population immunity (note- immunity not coverage) at above 93% to 95% (this is 1 -1/Ro, where Ro is the average number of secondary cases caused by each measles case in the absence of immunisation programmes). Ro for measles is believed to be between 15 to 20, but there are no good developing country data on this. Population density and mixing rates will affect the Ro. I hope this helps. Please let me know if you need any clarification. Kind regards Oz Dr Osman David Mansoor Scientist, New Vaccines, EPI unit World Health Organization Western Pacific Regional Office ________________________________________________ 3- My point of view regarding this topic is that most vaccination programmes do not yet give this second dose. I think that for the time being if there are measles cases in an area despite high vaccination coverage, we should confine ourselves to supplementary activities to strenghten routine vaccination. Many factors can influence vaccination. Tidiane Sidibe ________________________________________________ 4- There is no ready answer to Anil Varshney's inquiry. He states that measles vaccination coverage in his EPI programme is 96% and that the measles burden of disease is "low". Whether or not to add a second dose of measles vaccine into the program depends upon two main factors: the national goal for measles (i.e., mortality reduction, control or elimination) and current measles epidemiology. If the current burden of disease is "acceptable" to policy makers, then a single dose of measles vaccine with very high coverage may indeed be sufficient. Many countries using a single dose schedule with high coverage, however, have found that the periodic measles outbreaks which occur due to the accumulation of susceptibles (unvaccinated children plus those children who were vaccinated, but failed to respond immunologically [primary vaccine failures]) to be unacceptable. It is important to keep in mind that although measles vaccine is highly effective, vaccination coverage is not the same as population immunity. Assuming that measles vaccine when administered at 9 months of age is 85% effective, 96% vaccination coverage will result in a population immunity of 81.6%. Therefore, even a strong immunization program using a highly effective vaccine will leave over 18% of each birth cohort susceptible to measles. Over time the number of susceptible children will increase, thus increasing the risk of a measles outbreak. Measles virus is very infectious. To prevent periodic measles outbreaks, the World Health Organization recommends that all children should be provided with a "second opportunity for measles immunization". Most children who fail to respond to the first dose will become immunized when vaccinated a second time. The second opportunity for measles immunization may be provided through either routine immunization services or through supplemental immunization activities. The final decision of whether to routinely provide a second opportunity for measles immunization needs to be based upon national disease control targets and current burden of disease. Additional information on this subject can be found at: http://www.who.int/vaccines-documents/DocsPDF01/www667.pdf http://www.who.int/vaccines-documents/DocsPDF01/www573.pdf Strebel P, Cochi S, Grabowsky M et al. : "The unfinished agenda of measles immunization". J Infect Dis 2003;187:S1-S7. Please let me know if you have any further questions on this subject. Best regards. Brad Hersh ------------------------------ Bradley S. Hersh, M.D., M.P.H. 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