Vietnam is preparing for as many as 30,000 new Covid-19 cases as the more infectious variant of the virus first detected in the UK spreads through the north of the country.

The country’s health ministry reported nine new coronavirus cases on Friday in four northern provinces and the capital Hanoi, bringing the total number of local transmissions reported this week to 91.

The outbreak has jolted Vietnam, which had reported no local infections in almost two months. The country’s efforts to contain Covid-19 have been hailed as a global success.

It also coincided with a high-profile congress of the ruling Communist party, just ahead of the Tet lunar new year holiday, when many Vietnamese travel home to see their families.

The outbreak, first reported in the northern provinces of Quang Ninh and Hai Duong, is the largest in Vietnam since the country reported its first cases of the disease in January 2020.

Vu Duc Dam, deputy prime minister and head of the Covid-19 task force, said authorities should prepare for 30,000 cases, state TV channel VTV reported.

After word of the infections emerged on Thursday, Prime Minister Nguyen Xuan Phuc called a meeting on the sidelines of the party congress and issued a directive calling for lockdowns in two areas where the first new cases were detected.

Nguyen Thanh Long, health minister, said the number of Covid-19 patients was expected to rise and that the ministry had ordered three field hospitals to be set up in Hai Duong.

Vietnam closed its borders to most international travellers in March. It is unclear how the new B.1.1.7 variant of the virus, which is more contagious than the original strain, reached the country.

A woman who travelled from Hai Duong to Japan for work this month tested positive for the variant on arrival in the country, Vietnamese state media reported this week. Authorities sealed off Chi Linh, her home city, and began conducting mass testing and contact tracing in the areas where she lived and worked.

Vietnam’s ability to contain its first wave of infections, thanks to vigorous contact tracing, strict quarantine requirements and border controls, has allowed most aspects of normal life to resume.

The country last year attracted new investments from LG Electronics, the South Korean technology company, and Taiwanese manufacturer Pegatron as well as other companies seeking to diversify their supply chains amid growing US-China trade tensions. Vietnam’s economy grew by just over 2 per cent in 2020.

Australia’s Lowy Institute, a think-tank, this week ranked Vietnam second in the world, just behind New Zealand, in terms of its success at managing the pandemic.

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Additional reporting by Pham Hai Chung in Hanoi