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More on MMR and autism

UK study
California study
Comment
If you came to a small Oxfordshire village during the day and counted the number of lights that were on, you would find few. Come at night and you would find many. One conclusion might be that putting the lights on has caused it to become dark. Part of the MMR-autism debate ( Bandolier 84 ) has involved a similar observation, that autism was rare and now is common, and MMR was rare and now is common. Presto, MMR causes autism!

It may be a bit more complicated than that. Two new studies [1,2] have examined the temporal relationship between MMR vaccination rates and autism. Autism is on the increase despite high and stable MMR vaccination rates.

UK study


One study from Boston is based on the UK General Practice Research Database. It identified 305 children (254 boys) aged 12 or younger whose diagnosis of autism was first recorded between 1988 and 1999. The peak age of first diagnosis was at years 3 and 4, but with a substantial number being diagnosed at six years or older. The number of cases and incidence of autism increased substantially and constantly over the period (Figure).

Figure: Total number of cases of autism and incidence per 10,000 person years in UK



The study included a detailed analysis of 114 boys born in 1988-1993 who had a first recorded diagnosis of autism at ages 2-5 years. For them the four-year risk of diagnosed autism rose from 8 per 10,000 for boys born in 1988 to 29 per 10,000 for boys born in 1993 while the MMR vaccination rate was constant at about 97%.

California study


The California study used data from 21 Regional centres covering all of California, and for the years 1980 to 1994. MMR immunisation rates by two years of age were about 72% before 1988 and about 82% afterwards, with the same preparation used since 1979. During this time the number of cases of autism, about 200 in 1980, increased inexorably to about 1200 by 1994. The trend for increasing autism in California persisted long after the introduction of MMR vaccination, and was not affected by a modest increase in immunisation rates in the mid 1980s.

Comment


This continuous upward trend in autism dating from the late 1970s or early 1980s has been seen before in a study from North Thames [3]. We now have three studies, all showing this inexorable rise irrespective of whether immunisation rates are high and uniform, or pretty high and getting higher.

MMR has been available and been used for years. None of these studies supports, and all refute, that autism is caused by MMR vaccination, or that MMR vaccination is responsible for a major number of cases of autism.

What these studies all confirm is that autism, especially among boys, is on the increase. We don't know why. There is a plan for a further large case-control study using the UKGPRD [4]. It is predominantly to examine the link between MMR and autism, but may provide clues about links with environmental or other factors linked with autism.

References:

  1. JA Kaye et al. Mumps, measles, and rubella vaccine and the incidence of autism recorded by general practitioners: a time trend survey. BMJ 2001 322: 460-463.
  2. L Dales et al. Time trends in autism and in MMR immunization coverage in California. JAMA 2001 285: 1183-1185.
  3. B Taylor et al. Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association. Lancet 1999 353: 2026-2029.
  4. L Smeeth et al. A case-control study of autism and mumps-measles-rubella vaccination using the general practice research database: design and methodology. BMC Public Health 2001 1:2 ( www.biomedcentral.com/1471-2458/1/2 )
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