It is a fact. There is no evidence for the claim that H1N1a vaccine causes GBS.
The truth is that there are people that contracted GBS after vaccination...
That's true. There are 4 cases of CBG occurring in persons after receiving the H1N1a vaccine.
... no one at all can say with 100% assurance that the vaccine did not cause or contribute to it. And that is evidence.
That contradicts what you said earlier. First you claim that there is evidence that H1N1a causes GBS. Now you claim no one can say that the vaccine did or didn't cause GBS.
And yes, you CAN say whether or not the vaccine, or any condition, causes GBS. You've already indicated no objection to the use of statisical control groups and statistical study groups, so let's see what happens:
GBS occurs normally in the general unvaccinated popilation at the rate of 2 people per 100,000. Therefore, in the vaccinated population, one would expect 2 in every 100,000 people to likewise contract GBS. The rate was 4 in 600,000, which is actually a little less than the expected rate.
The conclusion is that GBS doesn't occur in H1N1a recipients any more than it does in the general unvaccinated population. That demonstrates that the H1N1a vaccine does not have any impact on whether a person contracts CGS. Since no cause and effect relationship can be demonstrated, it is a statement of fact to say that there is no evidence for the claim that H1N1a causes GBS.