GENEVA — COVAX has signed agreements with two Chinese pharmaceutical companies to buy more than half a billion of their COVID-19 vaccines by the first half of next year, Global Alliance for Vaccines and Immunization (Gavi) announced on Monday. Under the agreements, Chinese vaccine makers Sinopharm and Sinovac will begin to make 110 million doses immediately available, according to a news release from Gavi, a public-private global health partnership that is co-leading COVAX, a worldwide initiative aimed at distributing vaccines to countries regardless of wealth. The agreements came at a time when "the Delta variant is posing a rising risk to health systems," Gavi said in the statement. Gavi has the option to purchase a total of 170 Sinopharm vaccines and 380 million Sinovac vaccines, Gavi said. China has already sent millions of vaccine doses around the world as part of President Xi Jinping"s vision to make the country"s vaccines a "global public good." As of June 30, Sinovac had delivered more than 1 billion doses worldwide, the company"s chairman and chief executive Weidong Yin said in a statement about the COVAX deal. "Our mission at Sinovac is to supply vaccines in an effort to eliminate human disease," he said. The new agreements were welcomed by Dr. Seth Berkley, the chief executive of Gavi, who said the vaccines were an example of Gavi"s strategy to ensure there were "options in the face of constraints such as supply delays." COVAX"s rollout fell behind schedule earlier this year, after a coronavirus crisis in India meant the country"s biggest vaccine Monday"s announcement comes as Chinese vaccines face growing scrutiny over their efficacy, particularly against the swift-spreading Delta variant. Thai health authorities announced on Monday that health care workers would receive a booster shot of either AstraZeneca or Pfizer, after 618 of more than 677,000 medical personnel who received two doses of Sinovac tested positive for Covid-19. Of the 618 who tested positive, only two became seriously ill, including one nurse who died. "Despite that, all vaccines have been proven to be efficient in preventing hospitalization and death," Ministry of Foreign Affairs official Pensom Lertsithichai said at a briefing Monday, adding that medical workers had high exposure to Covid-19 which could have contributed to "vaccination failure." Officials in Singapore said last week they would exclude Sinovac shots from the city-state"s total vaccination count due to inadequate efficacy data for the vaccine, especially against the Delta variant, Reuters reported. Both Sinopharm and Sinovac vaccines have been validated by the World Health Organization (WHO) for emergency use. So far, trials show Sinopharm and Sinovac have a lower efficacy against Covid-19 than their mRNA counterparts. In Brazilian trials, Sinovac had about 50 percent efficacy against symptomatic COVID-19, and 100 percent effectiveness against severe disease, according to trial data submitted to WHO. Sinopharm"s efficacy for both symptomatic and hospitalized disease was estimated at 79 percent, according to WHO. Vaccines from both Pfizer/BioNTech and Moderna — which have also signed deals with COVAX — are more than 90 — effective against symptomatic COVID-19. COVAX has legally-binding contracts with manufacturers for about 3.8 billion doses — and 1.9 billion of those are forecast to be available for supply by the end of the year, according to COVAX"s most recent global supply forecast. According to UNICEF"s vaccine supply dashboard, 107.5 million doses have been shipped through COVAX to 135 countries. — CNN