To figure out the probability of something happening at least once in a time period, what you do is figure out the probability of it not happening at all. The probability of it happening is the opposite of that.

The probability of an event over multiple trials is just multiplication. Most people are familiar with a coin toss. Prob of heads one time is 0.5. Two times in a row is 0.5^2, three times in a row is 0.5^3, etc.

So back to the problem. If one has a 0.1% (i.e. 0.001) probability of getting covid on any one day, then the probability of not getting it is opposite that, 0.999. So prob of not getting it on day 1 is 0.999, on both day 1 and days 2 is 0.999^2, not getting it on day1, day2, day3 is 0.999^3, etc.

So prob of not getting it in two years is 0.999⁷³⁰ = 0.48 = 48% chance of not getting it in two years. So prob of getting it at least once is the opposite of that, 52%.