Just like the flu virus, they mutate.
So another vaccine needed. 😲
The case found in Hong Kong was a 36-year-old man who had a negative PCR test before flying from Hong Kong to South Africa, where he stayed from 22 October to 11 November. He tested negative on his return to Hong Kong, but tested positive on 13 November while in quarantine.
Very evasive... maybe booster shots reduced to three to four months