Prevention of Measles Virus Infections

G. Prevention

With no animal reservoir, it must be possible to eradicate the virus through a controlled vaccination campaign. In the USA, where vaccination of all children is required before commencing school, case reports have fallen by over 99% but eradication has not been achieved. The following vaccines are available

1. Inactivated Vaccine - this vaccine was intended for use in young children less than 1 year of age who are most prone to severe complications. It was thought to be advisable to avoid the use of a live vaccine. It was found that at least 3 doses were needed to elicit a protective antibody response but the antibody levels soon waned. This leave the vaccinees open to attack by the natural virus. In some cases, the nature of the partial immunity led to serious hypersensitivity reactions to infection (Atypical measles). The exact mechanism is still uncertain but it was thought that the vaccine lacked an important antigen of the virus and thus immunity was not complete. In view of the above and the fact that antibody levels decline rapidly after administration of the killed vaccine, live vaccination is now generally recommended and individuals previously immunized with the killed vaccine should be reimmunized with the live vaccine. The killed vaccine has now been withdrawn.
   
2. Live vaccine - live vaccines are now usually used. The seroconversion rate is 95% and the immunity lasts for at least 10 years or more, possibly lifelong. The virulence of the attenuated strain now in use is so low that encephalitis has only been noted in 1 in 1 million recipients. SSPE has been reported in children given the live vaccine. However, the rate is lower than that following natural infection. Therefore the vaccine is safe for use in very young children. The live vaccine is now incorporated as part as the MMR vaccine. As vaccine-induced measles antibody develops more rapidly than following natural infection, MMR vaccine can be used to protect susceptible contacts during a measles outbreak. To be effective, the vaccine must be administered within three 3 days of exposure. If there is doubt about a child’s immunity, vaccine should be given since there are no ill effects from immunizing individuals who are already immune. Immunoglobulin should be given to those for whom the vaccine is contraindicated.

The vaccination programme has been most effective in the USA, where measles immunization is compulsory. The incidence rate has also declined dramatically in the UK but without the rigorously pursued vaccination as practiced in the US, it is not likely to be as effective as that in N. America. In the third world, malnutrition aggravates measles infection and there are 900,000 measles related deaths per year. Vaccination in these areas has failed to yield dramatic results. The problem is that the vaccine is usually given at 12 months of age (it should not be given in younger individuals because the presence of maternal antibodies may lead to a poor response.) but infection in these areas often occurs earlier in life. Vaccination should therefore be performed on younger children than in the developed world. However, this must be balanced with the fact that the success rate is lower in younger children (50-75% in 6-month-old-children as opposed to 95% for 12-month-old children.). Measles is highly infectious and has a very high attack rate and thus it would be extremely difficult to eradicate the virus altogether through vaccination.

Management of Outbreaks

Measles outbreaks are most deleterious in wards with immunocompromised children or adults e.g. children with leukaemia and bone marrow transplant recipients. Measles is definitely as dangerous as VZV in that setting. HNIG should be given to all severely immunocompromised children irrespective of their immunization status since it has been reported that severe measles infection can occur in those who had been immunized and had a documented low-level antibody response. Therefore, the routine screening of children for measles antibody before admission is probably unjustified since there would be no difference in the management. The same argument applies to the screening of patients for immunity before the administration of HNIG. The use of live-attenuated vaccine for postexposure prophylaxis is contraindicated. The same protocol applies to immunocompromised adults who come into contact with measles. Immunocompetent children under 12 months in whom there is a particular reason to avoid measles, such as a recent severe illness, can also be given immunoglobulin. MMR vaccine should then be given after an interval of at least 3 months, at around the usual age.