That doesn't matter much when there is no strong evidence that to have had COVID guarantees immunity[0] because being asymptomatic now doesn't mean they will be asymptomatic in the future, just South Korea has reported 116 people re-infected[0]
It's much more likely those in Korea received false negative test results earlier on. Let's say that was the case, though, 116 out of 2.5 million rounds to zero and is of no practical consequence to herd immunity or the potential effectiveness of a vaccine. All we need for either is 60-70% of people ending up with immunity -- so far your number is 0.0046%