Alex is a 32-year-old man who is scheduled to travel to a country where hepatitis a is endemic. he leaves in 3 weeks and will be gone a month. he received his first dose of hepatitis a vaccine today. alex should be counseled to return for his second dose of hepatitis a vaccine:

Since Alex is taking a hepatitis a vaccine, he should receive two doses of which is required with the vaccine. The interval of the two doses is six months apart. Therefore, if he already had receive his first dose, he will receive the second dose after six months starting from after the day that he has been injected.

It's cell theory. easy name i would think.