I was wondering last night, approx 10% of those that have had CP get it again, so natural immunity is about 90% effective.
www.nhsdirect.wales.nhs.uk/encyclopaedia/c/article/chickenpox/
In a study, up to 13% (around one in eight) of people diagnosed with chickenpox reported that they had had the condition before.
US studies conducted from 1995 to 1999 found that 4.5 to 13% of people diagnosed with chickenpox reported that they had had the condition before.
Other US studies have found that after having chickenpox, some people do not develop the antibodies needed to protect them against reinfection. For example:
from a report in 1996: tests on adults who reported having chickenpox showed that 97-99% of them had developed antibodies
from a report in 2007: tests showed that after having chickenpox, three quarters of children aged one to four had developed immunity, compared with all adults aged 20 to 29
Experts agree that if you have already had chickenpox, it is very likely that you will have developed antibodies.
Do you know how effective the vaccine is?
www.nhs.uk/Conditions/vaccinations/Pages/chickenpox-vaccine.aspx
It has been shown that nine out of 10 children vaccinated with a single dose will develop immunity against chickenpox. A two-dose schedule is recommended, as it gives an even better immune response.
The vaccination is not quite as effective after childhood. It's estimated that three-quarters of teenagers and adults who are vaccinated will develop immunity against chickenpox.
75% protected in those taking vaccine in teenage years to 87% at worst with chickenpox. Though the chickenpox virus comes with small number of serious cases.
But if vaccination in Canada is routine like I think USA is then herd immunity plays a part in reducing risk of contracting it anyway.