Tuesday, December 22, 2015

The Curious Case of Declining Measles Infections

So I had an interesting conversation recently with Destroyed by Science and The Technical Skeptic on the topic of measles death statistics. We were trying to reconcile the US death rate and those published by the WHO. Eventually it dawned on us what the big differences between the two numbers were, so I thought I'd write up a post to discuss it.

The US death rate is 1-2 deaths out of 1,000 cases (0.1 to 0.2%). This makes the measles seem like it really isn't that bad. However, the worldwide death rate is more troubling. Out of 266,701 reported cases there were 145,700 deaths for a rate of 54.6%; this was the source of our confusion. You don't have to be a mathematical genius to realize that 54.6% is not the same as 0.1%. Digging deeper the reasons for this difference becomes quite clear.

A big reason for the dichotomy is because the WHO data is worldwide and is a total. The number of cases and the number of deaths are much lower in developed nations compared to developing nations.

Number of Reported Measles Cases with onset date from May 2015 to Oct 2015 (6M period). 

Because most of the cases are happening in places where the healthcare systems are not as developed, fatalities from the virus are much higher. A big part of this is pneumonia, which occurs in 1 out of every 20 children who contract measles, either from the measles virus itself or as a secondary infection from other viruses or bacteria. Pneumonia is in fact the leading cause of death with the measles. In developed nations hospital care, and antibiotics for cases of bacterial pneumonia, is available for patients which reduces the impact of pneumonia whereas this is not the case in developing countries.

Another factor that reduces the impact of measles in developed nations is the improved nutrition whereas developing nations suffer from malnutrition and deficiencies. In particular, Vitamin A deficiency increases the fatality rate of measles. One of the areas with high numbers of measles cases, SE Asia, also suffers from Vitamin A deficiency despite the availability of a source for Vitamin A that would help alleviate the deficiency: Golden Rice.  This makes opponents of Golden Rice, like GreenPeace, doubly damned as they are not only allowing blindness from Vitamin A deficiency to languish but risking the lives of millions of children to complications from measles.

The final factor that impacts the death rate is the vaccination rate. In most developed nations, or at least until recently, vaccination rates for measles were above 90% which reduces the severity of the disease and reduces the death rate if at least one vaccination dose has been administered and of course more doses offer a higher degree of protection. This has been changing recently with the WHO, The Gates Foundation and others pushing for measles vaccination in developing nations.

Immunization coverage with first measles vaccine dose in infants, 2014

This push for immunization has resulted in dramatic results that cannot be disputed: measles cases worldwide have dropped dramatically since the vaccination program was initiated.

Trend of measles cases compared to vaccination rates worldwide.

So the take away message here is that you can't use the rates in developed nations as the sole determinant for how bad measles can be. After all, measles has a long and brutal history of killing large numbers of peoples exposed to it for the first time. Measles is highly contagious and those exposed to it without the protective antibodies from a vaccine, for example, are very likely to get infected. I won't rest easy around measles until it has gone the way of smallpox or rinderpest (a relative of measles). We also learned that trying to do math late at night is a recipe for disaster.

No comments:

Post a Comment