PHNOM PENH, Cambodia—Cambodia on Friday began a fourth round of vaccinations against COVID-19 in response to the Omicron variant, with high-risk groups among the first to receive the additional boosters. Front-line medical staff and members of the armed forces were among those lining up at hospitals and clinics. Government ministers, including Prime Minister Hun Sen, also received booster doses on Friday. Almost 90 percent of Cambodia’s 16 million people have had at least one dose, over 85 percent have had a second shot and more than 27 percent have had a third, according to the government. But the Chinese Sinovac and Sinopharm vaccines have been found to have less efficacy than others against the Omicron variant, especially with the passage of time, so the fourth round of high-priority vaccinations is using Cambodia’s more limited supply of Pfizer vaccines. Neighboring Thailand, which like Cambodia was largely reliant on the Chinese vaccines …