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Tuesday, 21 February 2012

Measles - The Moose is Loose

PHAC recently issued a travel advisory on measles globally as a reminder to travellers to be sure about immunity. PHAC travel advisory.    International travel has become the major contributor to importation of measles into Canada. 

Then as this posting went up, a report of spread of measles in the US related to the Superbowl.   Measles and Super bowl with at least 14 cases so far and more spread is likely given the number exposed

Great advances have been made in controlling measles in Canada, so much so we often forget the devastation that the disease reaped havoc prior to the ‘60s when 250,000-400,000 cases occurred in Canada annually.  Measles remains the number one vaccine preventable cause of death internationally with some 150,000-200,000 deaths per year.  The embedded graph provides the Canadian view of measles control, something that is replicated in most developing countries.

2002 the Americas were declared as measles free for indigenous spread.  Already in 2012 there have been nearly a 1000 cases in the Americas alone PAHO weekly reporting 

While it is difficult to be sure on numbers, Canada had 750 cases in Quebec in 2011 Quebec measles outbreak update, 79% of which were underimmunized.  BC in the wake of the 2010 Olympics saw 82 cases.   Both outbreaks sparked by importations into the country and then sustained local transmission.  Both outbreaks speak to the vulnerability of Canada that despite great immunization programs, sustained transmission can and does occur. 

The reproductive number of measles is estimated at 12-18 – meaning every case could potentially infect 12-18 non-immune persons.  Of course the propagation of the disease will occur when the probability that each case contacts at least one other non-immune person.  When population immunity levels begin to drop to where one in 20 is susceptible then the likelihood that each case will transmit to one or more people becomes more likely and propagation will occur.   (Simply put, but a close estimate).   Even the best 2-dose coverage levels in Canada are just over 90%, with about half of the underimmunized having had one dose of vaccine. 

Despite the international efforts to contain measles, dissemination and propagation continue to occur.   It remains a public health emergency and requires immediate mobilization to contain importations – just that the event would appear to be getting more frequent and the consequences are not just a few cases, but into the tens and hundreds before control is achieved.   On the positive side, the first decade of this century saw global measles deaths reduced by nearly 80% - short of the objective, but a huge improvement.  

More is available on measles in Canada at PHAC measles site, but as is typical of PHAC, that sense of declining success and building urgency is lacking.   

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