A medical equipment group on Facebook has been recording at least 20 such posts a day, with the test kits ranging from VND100,000 ($4.35) to VND800,000.

They are often advertised as having an accuracy rate of over 92 percent. Each kit can test two persons in 15 minutes each.

Chinese equipment cost the least, Vietnamese kits cost VND200,000, while those from Germany or the U.S. are priced at up to VND800,000.

Rising demand from factories, companies and residents is driving sales, Le Xuan Tinh, a seller in Ho Chi Minh City, said.

HCMC has been recording thousands of new Covid cases daily, while Hanoi began a stringent 15-day social distancing campaign on Saturday.

Even the same brand of kits could be sold at different prices. Hong Hanh, a seller in Hanoi, said the average price of a Humanis kit, certified by the Ministry of Health, is VND250,000, but the same products with unclear origins are sold at 20 percent less.

Prices change every day, and buyers have to wait for at least 15 days if they want large quantities, she added.

But experts advise people against buying unauthorized kits to test themselves.

"There are many products with unclear origins and do not guarantee accuracy," Tran Dac Phu, a consultant at the Vietnam Public Health Emergency Operation Center, said.

Rapid test kits are most accurate for two to seven days after someone contracts the novel coronavirus.

The HCMC Department of Health gives the same advice, pointing out that incorrect negative results would lead to the spread of the disease.

Vietnam has had over 97,000 cases in the latest Covid wave starting April 27.