Man found to have been shedding virulent strain of polio for 30 years | Science | The Guardian
http://www.theguardian.com/science/2015 ... r-30-years

If it is a new strain, why are people who are vaccinated against the old one still protected? I thought vaccinations were usually against specific microbes?

It's not a new strain as such, it mutated from the weakened virus given the individual when he was vaccinated as a child. It appears to have been able to do this because he has an immune deficiency which prevented his immune system from killing the virus in his gut. The mutation appears to have increased the damage it can do compared to the weakened strain in the old vaccine but doesn't appear to have induced a change which makes it less susceptible to an intact immune response from killing it off.

Some vaccines very definitely are directed against a specific from of a microbe - the flu vaccines for example, but that's not necessarily the case for all vaccines. It depends how the disease mutates and evolves and also how many variants there are.
Things like flu and also HIV are very difficult to produce a general vaccine for because the mutate quickly and different strains branch off from the original virus to create tens, hundreds or even thousands of sub-strains that are all sufficiently different to make targeting one strain ineffective against the others. That's why, for example, for flu season the WHO basically takes an educated guess at which strains are likely to be a problem that year and then directs drug companies to manufacture vaccines effective against the strains expected to be most dangerous that year. They aren't always right which is why some people still get flu despite having been vaccinated - the vaccine didn't protect against a specific strain that they caught.

In the case of the polio virus it appears that whatever mutations have so far surfaced haven't seriously affected the ability of the vaccine the direct the immune system to correctly identify and kill the virus. At a guess that probably means the mutations 'in the wild' so far haven't seriously altered the surface structure of the virus so the immune system, once it knows what it's looking for, can easily mop up the virus.