The proposal is part of efforts to achieve the global goal of eradicating polio by 2018. But it would also be one more barrier for migrants — especially those fleeing armed conflict or human rights violations. These are not the ideal conditions under which to queue for a polio shot and get the paperwork done.
Failure to control the disease often reflects a failure of governance, sometimes as a result of political instability. For instance, although Syria was declared polio-free in 1999, it has reported 27 cases since a 2013 outbreak because the civil war led to a drastic cut in vaccination coverage. From there, the virus spread across the border to Iraq. Similarly, an outbreak in Pakistan spread to Afghanistan, and an outbreak in Cameroon has affected Equatorial Guinea.
All these countries are refugee origins or destinations. Other countries where the WHO found the poliovirus are Ethiopia, Israel, Nigeria and Somalia. 
Getting a polio-vaccination certificate may be difficult amid conflict or other violence. And it is questionable whether a certificate issued to a desperate migrant in such a scenario will be genuine.
“Although Syria was declared polio-free in 1999, it has reported 27 cases since a 2013 outbreak because the civil war led to a drastic cut in vaccination coverage.”
Requiring a certificate could also distort public perceptions and, by extension, policies — inadvertently encouraging migrants to be viewed as a potential health threat. Media reports in various parts of the world suggest that recently, especially in the aftermath of the global financial crisis of 2008-2009, much hostility has been drummed up against migrants. In such a climate, a seemingly official tag of being a health threat is the last thing that a migrant or a refugee needs.
Travel restrictions do help in disease control, but only in conjunction with better interventions, such as access to high-quality vaccines, for people coming from or going to countries with polio.
And considering the practical issues involved in getting a vaccination certificate in countries that face difficult situations such as conflict, the WHO and other UN bodies should make it easier for migrants to get clearance to enter another country. Or they could explore alternative solutions, such as vaccination facilities for refugees and others forced to migrate. At the same time, UN agencies should work to counter the negative perceptions of migration that might come from such disease-control interventions.
Max Martin is a doctoral candidate at the University of Sussex, United Kingdom, researching on climate-related migration. The views expressed are his own.
References World facing polio health emergency (BBC, 5 May 2014)
 To protect progress, WHO declares polio spread public health emergency of international concern (Global Polio Eradication Initiative, May 2014)