### instruction:
If you are a doctor, please answer the medical questions based on the patient's description.

### input:
A few years ago I got the typhoid fever shot because I was going to be traveling to some poorer countries. 1 month after I had the shot, I went on vacation and became violently ill. The doctors told me it was typhoid fever. I was tested, and I responded positively to SALMONELLA O, TYPE Vi POSITIVE A with somatic(O) antigens. My question is, how do I know for sure that I did have Typhoid fever (and that the positive test wasnt the result of my earlier vaccination)? Wasnt my vaccination supposed to prevent the Typhoid? What are the chances someone can get Typhoid even though they were vaccinated? Does this mean I can get typhoid fever again (do I need to be vaccinated again?)

### output:

Hello,first, the vaccine does not confer 100% protection, so getting sick cannot be excluded. The diagnosis of typhoid fever is not based on a positive H and O because this test is considered obsolete. And the reason is that a patient may have other pathogens that cross-react and give positive results, so an over diagnosis of typhoid fever would occur if we relied on this test only. The most accurate way to establish the diagnosis is by culturing the organism. Stool cultures is the most feasible strategy, but bone marrow specimen can be used as well. The vaccine efficacy is not very well studied, but it can be about 70%. It declines rapidly with time. Three years after vaccination It's supposed to be lower than 50%. If this was typhoid fever indeed then this acts as a booster for your immune system. So if you're going to travel again in the future then perhaps you should receive vaccination again, particularly if more than a couple of years have passed since the last vaccination. Kind Regards!