booster shot noun
plural ∼ shots [count] chiefly US : an extra amount of a substance (called a vaccine) that is injected with a needle into a person or animal to help protect against a particular disease
▪ He had to get two booster shots at his latest checkup. ▪ a tetanus booster shot —called also booster
▪ He had to get two booster shots at his latest checkup. ▪ a tetanus booster shot —called also booster
