Guest blog post from WH
The What Doctor’s Don’t Tell You website is up with a new scare story about MMR vaccine. The scare story itself has been debunked but let us take a look at the advantage that MMR vaccine provides by preventing measles and what will happen if the vaccine coverage decreases.
There has been a study by the good people of the Health Protection Agency examining the potential for measles transmission in England. The study concluded that even allowing for considerable underreporting of vaccine coverage, several districts have the potential for sustaining measles transmission.
The study was a well-conducted analysis; the methods were rigorous and well-reasoned and it deserves a closer look. In the study, the data about vaccine coverage was collected from COVER (Cover of Vaccination Evaluated Rapidly) programme between 1995 and 2002 and from primary care trusts between 2002 and 2006. The authors had accounted for missing and anomalous data. They had also performed a sensitivity analysis to estimate the under-reporting of vaccine coverage which came out around 24%. So the data collection technique of the study was robust.
Then the authors found the proportion of the population susceptible to measles according to age group using the following technique –
Proportion susceptible = 100% * proportion with 0 doses + 10% * proportion with 1 dose only + 1% * proportion with 2 doses.
This is a very good estimation when we consider the effectiveness of the MMR vaccine.
The potential for the transmission of measles and the size of the outbreak was estimated using the effective reproduction number. Effective reproduction number (R) is the average number of secondary cases per primary case seen in a population with an infectious disease. Secondary cases are all those cases that come in contact with the primary case and subsequently develop the disease within one incubation period. The value of the effective reproduction number determines if an outbreak will be sustained. If R>1, the outbreak will perpetuate, if R<1, the outbreak will die out and if R=1, the number of cases will remain same. Vaccination prevents the occurrence of an outbreak by keeping R below 1. More information about effective reproduction number is available here. [Note: link goes to PDF] The estimation of R was done using the Next Generation Matrix (NGM). More about NGM is here.
Here is a graph showing the percentage of children susceptible to measles prepared from the data available in the study –
As much as 27% of the children in the pre-school age group are susceptible to measles. In their analysis, the authors found that 4 Strategic health Authorities (SHAs), all in London has the potential for measles outbreak with the highest R value in South West London. They also estimated outbreak sizes in case of a possible outbreak of measles. Here is a graph showing outbreak sizes considering 30% under-reporting of vaccine coverage –
When we add the complications of measles to this, the picture looks grimmer. More detailed analysis is available in the paper.
If the vaccine coverage remained stable since 2004/05, the size of the R value still continues to increase each year raising the potential of an outbreak and increasing the size of a possible outbreak.
It is a ticking time-bomb unless measures are taken to increase the coverage of MMR vaccine to reduce the R value below 1. Scare stories such as those of WDDTY have the potential to do much harm and absolutely no good.
Thanks to jdc for hosting.