After three years of COVID isolation, mainland Chinese residents are finally able to go abroad without having to quarantine when they get home. Many are using their first stops outside mainland borders to get injected with vaccines long available in much of the outside world. Clinics in Hong Kong, which opened its borders for quarantine-free travel on Jan. 8, say they are at full capacity serving mainland tourists with mRNA booster shots produced by Germany’s BioNTech. The Hong Kong branch