but by nine months later, the efficacy was not just zero, but negative. What ?
If people who have had the vaccine have smaller number of cases of COVID, the vaccine is said to have a positive efficacy.
If people who have had the vaccine have the same number of cases of COVID, the vaccine is said to have a zero efficacy.
If people who have had the vaccine have more of cases of COVID, the vaccine is said to have a _________ efficacy.
Fill in the blank...