Cambodia is offering booster shots using a Chinese vaccine. It's capital, Phnom Penh, is now offering Sinovac as a third shot. And several other regions and countries have launched their booster shot campaigns. Doctor Alice Tan, an internist at MizMedi Women's Hospital in Seoul, joins us via Skype to discuss the booster shot campaigns. She said that for the Pfizer vaccine, neutralizing antibody levels may be waning among those vaccinated six months ago or more, and the epidemiological data seem to show progressively increased numbers of breakthrough cases among those previously fully vaccinated. Thus, a booster shot is recommended for those at the highest risk of exposure, such as healthcare workers, and for those with immunocompromised states.