BANGKOK, Oct 18 (Reuters) - Thailand will stop using the COVID-19 vaccine of China"s Sinovac (SVA.O) when its current stock finishes, a senior official said on Monday, having used the shot extensively in combination with Western-developed vaccines. Thailand used over 31.5 million Sinovac doses since February, starting with two doses to frontline workers, high-risk groups and residents of Phuket, a holiday island that reopened to tourists early in a pilot scheme. In July, Thailand started inoculating people with Sinovac as a first dose followed by the Oxford University-developed AstraZeneca (AZN.L). Thailand was the first country to combine a Chinese and Western shots, a strategy its health officials said has proved effective. "We expect to have distributed all Sinovac doses this week," said health official Opas Karnkawinpong, adding the programme will switch to combining the AstraZeneca (AZN.L) vaccine with that made by Pfizer (PFE.N) and BioNTech (22UAy.DE). Thailand next year plans to buy 120 million COVID-19 vaccine doses in total and has already booked 60 million doses of AstraZeneca, a vaccine it manufactures locally. Thailand has said it will only procure vaccines effective against new variants. It has so far vaccinated 36% of the estimated 72 million people who live in Thailand and hopes to reach 70% by year-end. The country is forging ahead with a quarantine-free reopening plan next month of 17 provinces to vaccinated arrivals from low risk countries. Included will be destinations like Pattaya, Hua Hin, Chiang Mai and Bangkok. Thailand has recorded nearly 1.8 million cases and 18,336 fatalities overall, more than 98% in the past seven months.