Marvelous measles media and menacing measles myths

By Helen Petousis Harris 19/02/2014 6


There has been some great media reporting recently around the measles outbreaks popping up around the country, for example the Dom post  and a fun editorial in the Southland times 

Interesting how the focus is on the damage done to immunisation programmes as a result of the mischievous actions of one Andrew Wakefield . This, I think, has been partly driven by the recent rather nice visual interpretation of vaccine preventable disease outbreaks across the globe, courtesy of the Bill and Melinda Gates Foundation. Have a play with the dots and notice where they cluster, it is amazing how much disease there is in some developed countires with access to the best vaccines. While some anti vaxers think this is proof that vaccines don’t work it is simply a reflection of vaccine coverage and this is partly influenced by anti immunisation activities. Some folk have produced maps illustrating the relationship between communities that decline immunisation in high proportions and disease out breaks. This is a nice example from the Netherlands

mazelen_4-7

 

 

We can all pat ourselves on the back over our rather epic transformation from a country with almost the worst immunisation coverage to a country with some of the best immunisation coverage (go us!). Couple of important points, especially directed at the anti immunisation lobby who have trouble grasping this fairly simple concept.

This high vaccine coverage rate in NZ only applies to the young ’uns under about three years of age and diminishes as we go up in age group. Some age cohorts over the past 44 years have a vaccine uptake of around 50%. That is really, really bad and means that there are still many in our community who are unprotected. If you were born before the moon landing in 1969 then you are assumed to be immune as you will almost certainly have been exposed to natural infection.

If you or someone you know is 44 years old or younger ask this question: Have you had two doses of MMR or other measles containing vaccine?

  1. Yes – excellent you have around a 2% chance of getting measles should you meet it
  2. No, just one dose – Pretty good; you have around a 5% chance of getting measles should you meet it. Get another dose, its free.
  3. No – If you have not had measles disease (confirmed) then you have almost a 100% chance of getting measles should you meet it – and then passing it on to 12-18 of your closest acquaintances if they are not immune.
  4. No idea? Go find out.

 

The maths presented by the anti-vaccination lobby is strange. Here are their calculations

According to the 2012 report, the latest available, there were a total of 68 cases of measles reported for that year. Of those, 21 were in children aged under 15 months, which we will discount when assessing vaccination efficacy as they aren’t eligible for vaccination according to the New Zealand schedule (though one was reported to have had a single dose anyway!). That leaves us with 47 cases of measles in persons aged 15 months and over, 17 of whom had had at least one or two doses of a measles vaccine, with 20 reporting no vaccination and 10 with unknown vaccination status.

Surely, if the vaccine worked so well, we’d see the vast majority of those cases in unvaccinated people, rather than an almost 50/50 split? And while it could be argued that this does lean towards better odds being vaccinated than not given the higher population of vaccinated individuals, one has to question how exactly that works given the vaccine datasheet claims 95% efficacy against measles…

The last time I looked at the 2012 measles outbreak data from ESR (Weekly report 50, week 10 2012) there were 580 cases. Of these 24 were fully vaccinated and 356 were known to be unvaccinated.

 

Measles 2011- 2012

While I am tempted to point out that fewer than five percent were fully vaccinated this would be misleading and an error in logic on my part.

It’s all about using the correct denominator!! (number at the bottom of the fraction)

For a very crude estimate of vaccine effectiveness one would need to put the unvaccinated measles cases over the unvaccinated population and the fully vaccinated measles cases over the number of fully vaccinated people in the population. I will let you mull that over but I think you will find that the vaccine appears to be exceptionally effective.

..ok here is a super duper crude example using ESR data and some educated assumptions about vaccine uptake to illustrate the point Please note this is just an example to illustrate the basics and there are many other complexities that are normally taken into account when calculating vaccine effectiveness in the real world!!!:     Below is an example for ages 15 month to 3 years assuming a 91% coverage rate for the 18 month cohort of approximately 100,000.

Vaccine effectiveness % = 1- (Attack rate in unvaccinated/Attack rate in vaccinated) X 100

1-  {(45/9,000)/ (0/91,000)} x 100 = 1-0 X 100 = 100%

For the four to nine year olds assuming an average of 88% immunisation coverage:

1-     {(5/343,000) /  (77/46,800)} x 100 = 1- (0.000015/0.0016) = 1- 0.0094 X 100 = 99%

 

Hopefully this illustrates the point.

 

 


6 Responses to “Marvelous measles media and menacing measles myths”

  • tsk Helen. There you go letting facts get in the way of a solid emotional argument…

    😉

  • Thanks for the informative post. It’s really important to counter the fear that misinformation creates. Ultimately, the facts are so solid that the only way someone can deny them is to believe there is an enormous conspiracy theory.

  • ESR figures for measles for 2012 (taken from Notafiable Diseases Annual Report https://surv.esr.cri.nz/PDF_surveillance/AnnualRpt/AnnualSurv/2012/2012AnnualSurvRpt.pdf) “In 2012,68 cases of measles were notified (55 cases were laboratory confirmed), compared to 596 cases in 2011 (461 cases were laboratory confirmed)”. Maybe you were looking at the 2011 numbers accidentally, the weekly report to March 2012 could not have had the figures for the whole year anyway, and I cannot see your above table in that report, perhaps you were sent a different report from ESR that is not on the website? https://surv.esr.cri.nz/PDF_surveillance/MeaslesRpt /2012/201210MeaslesRpt.pdf I also note an error in the ESR weekly report (table 4) that says there were 3 measles cases for that week in under 15 month-olds but this figure has not shown up in the total number for that week.

    • My figures come from the Measles report for the 2011/12 outbreak. When there is an outbreak a report is issued weekly. I used the last report that was available when I last looked. The measles outbreak straddled 2011/12 and the weekly measles specific reports are more informative than the annual reports.

  • One person with measles can infect 12-18 people not already immune to the disease. Westlake Boys High School, on the North Shore of Auckland known for pockets of anti-immunisation campaigners, provides an excellent example of how one person infects many. Someone took measles to school, now they’re up to 20 cases and more are expected! 20 students off school with measles and at risk of developing serious disease complications. Other students banned from school during the measles outbreak because they haven’t been immunised with MMR and haven’t had measles disease yet. These kids have been lucky so far, their parent’s rely on socially responsible parents to vaccinate their children and reduce the amount of disease in our community. But the anti-immunisation campaigners didn’t bargain on herd immunity, or lack there of, overseas, one non-immunised traveller brought measles home to NZ just after Christmas last year and Auckland alone is now at 40 cases and counting upwards. I just hope that none of these young people with measles end up with a life long disability through complications from their disease.