Shanghai has announced its city-wide lockdowns will be gradually lifted from 1 June, after 15 out of 16 districts achieving "social dynamic Zero-Covid cases".

China's biggest city imposed its first lockdown in the beginning of April, amid a surge in Omicron cases.

In spring, Shanghai confined some 25 million people to their homes. It previously tackled growing infection rates through smaller localised lockdowns.

This typically meant individual residential complexes - each housing several hundred people - were locked down instead of the entire city. And for a while, it looked like this method was working.

Even when case numbers rose to nearly 1,800 in March 2021, Shanghai did not impose a full lockdown.

By comparison, Xi'an, which is home to nearly 13 million people, sealed the entire city after less than 100 cases in December 2021.

The city of Yuzhou, in Henan province, locked down more than 1.1 million people after just three confirmed Covid cases.

But in late March, Shanghai began seeing huge numbers - with more than 2,500 cases recorded on 27 March alone.

The city decided to announce a phased lockdown, firstly in its eastern district, and then its western district days later.

But as cases continued growing, Shanghai officials abandoned the idea of keeping half the city open. A city-wide lockdown followed on 3 April.

Prof Martin Hibberd at the London School of Hygiene and Tropical Medicine thinks that Shanghai's previous approach was not suited to the Omicron variant, which is far more transmissible.

"Localised lockdowns of complexes are unlikely to restrict social interactions sufficiently, with such a large amount of asymptomatic transmission," he says.

The main reason is its importance for the Chinese economy.

Shanghai is responsible for more than 3% of China's GDP, and more than 10% of China's total trade since 2018.

Airports in Shanghai have also been responsible for bringing in nearly half of the protective equipment and medicine that China needed in the early days of the pandemic, according to local news outlet Caixin.

In 2020, cargo flights into Shanghai Pudong International Airport accounted for 3.4 million tonnes of goods - a million more than the airports in the cities of Beijing, Guangzhou and Shenzhen combined.

Studies from the Chinese University of Hong Kong show a two-week lockdown in megacities like Beijing or Shanghai could cost China 2% of its monthly GDP.

China's monthly GDP in 2021 was 9.5tn yuan ($1.4tn) on average, so the country would stand to lose about 190bn yuan ($29.8bn) for each week the lockdown continues.

Prof Hibberd suggests that these tough measures in Shanghai still may not be enough to tackle the Omicron outbreak.

Instead, he says, the highest priority to mitigate the effects of Covid should be to encourage everybody, "especially vulnerable people, to get vaccinated".

China has given more than 3.3 billion doses of Covid vaccines and vaccinated over 88% of its entire population.

But vaccine rates among people over the age of 80 - who are among the most vulnerable - remain a lot lower than other age groups.

The Chinese National Health Commission now says that vaccinating elderly people should be the priority task for local authorities. It has also approved clinical trials of two Covid-19 vaccines that use mRNA technology.

Until now, the country has heavily relied on domestically-produced vaccines which have struggled against the latest variant.

The Health Commission stated that "the current vaccines are still effective in tackling Omicron", but did not specify whether it was including Chinese vaccines.