The true scale of the epidemic caused by the new coronavirus in Hubei province has been thrown into doubt after the Chinese authorities reported more than 13,300 extra cases going back over an unknown number of days or weeks.

The World Health Organization (WHO) said the huge jump in cases in Hubei, bringing the total to more than 60,000 worldwide, was due to a change in the way Chinese authorities are counting them.

Cases where doctors have seen chest infection on a CT scan are now being classed as coronavirus rather than just those confirmed by a lab test result, leading to a 254 rise in deaths to a total of 1,370 since the outbreak began. All but two of the deaths have been in China.

In addition to the 13,332 extra cases, Chinese authorities on Thursday reported a further 1,820 laboratory-confirmed cases.

The WHO is now working hard to try to get further details of when the extra cases of what is now being called Covid-19 occurred, to have a true picture of how the epidemic has been developing in Wuhan and other cities in Hubei.

“Most of these [additional] cases relate to a period going back days or weeks,” said Dr Michael Ryan, the WHO’s head of emergencies. “It is retrospective reporting. It is largely due to how cases are being diagnosed and reported.”

On the positive side, it meant Hubei’s doctors would be able to report cases more quickly because they no longer have to wait for lab confirmation, said Ryan.

The political fallout from the outbreak also escalated on Thursday with the firing of Hubei’s party chief, the party chief of Wuhan and the head of China’s Hong Kong and Macau affairs office. Ying Yong, the new party chief of Hubei, came up through the ranks in Zhejiang – where President Xi Jinping, previously served as party secretary – and was also part of anti-corruption campaigns, the president’s signature initiative.

“This is clearly Xi’s move,” said Dali Yang, a professor of political science focusing on China at the University of Chicago. “The stakes are high and he needed time to find the right people for the positions to salvage the Hubei, Wuhan situation.”

Facebook Twitter Pinterest A man measures people’s body temperature at a roadblock in Guangzhou, China. Photograph: Alex Plavevski/EPA

On Wednesday, the state-run China Daily reported that a powerful Beijing official parachuted into Wuhan to supervise the fight against the virus had reprimanded local officials for failing to organise treatment quickly enough for people reporting to hospitals with symptoms.

Thursday’s jump in infections may have been another impetus for the purges. “I suspect Xi would have wanted the personnel change to project a sense that he is in control of the situation. The bad numbers undermine that message,” said Sam Crane, who teaches Chinese politics and ancient philosophy at Williams College in the US.

Prof Paul Hunter, a coronavirus expert from the University of East Anglia, said cases that in the past would only have been considered suspect would now fall into the confirmed bracket. Many would be Covid-19 cases that would have been confirmed if lab tests had been done, but the figures would also include some people who had pneumonia not caused by the coronavirus.

Quick guide What are coronavirus symptoms and should I go to a doctor? Show Hide What is Covid-19? Covid-19 is caused by a member of the coronavirus family that has never been encountered before. Like other coronaviruses, it has come from animals. The World Health Organization (WHO) has declared it a pandemic. What are the symptoms this coronavirus causes? According to the WHO, the most common symptoms of Covid-19 are fever, tiredness and a dry cough. Some patients may also have a runny nose, sore throat, nasal congestion and aches and pains or diarrhoea. Some people report losing their sense of taste and/or smell. About 80% of people who get Covid-19 experience a mild case – about as serious as a regular cold – and recover without needing any special treatment. About one in six people, the WHO says, become seriously ill. The elderly and people with underlying medical problems like high blood pressure, heart problems or diabetes, or chronic respiratory conditions, are at a greater risk of serious illness from Covid-19. In the UK, the National health Service (NHS) has identified the specific symptoms to look for as experiencing either: a high temperature - you feel hot to touch on your chest or back

a new continuous cough - this means you’ve started coughing repeatedly As this is viral pneumonia, antibiotics are of no use. The antiviral drugs we have against flu will not work, and there is currently no vaccine. Recovery depends on the strength of the immune system. Should I go to the doctor if I have a cough? Medical advice varies around the world - with many countries imposing travel bans and lockdowns to try and prevent the spread of the virus. In many place people are being told to stay at home rather than visit a doctor of hospital in person. Check with your local authorities. In the UK, NHS advice is that anyone with symptoms should stay at home for at least 7 days. If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home.

“The issue remains how are we going to be able to say what is happening with the trajectory of the outbreak when the cases definitions change midway through the epidemic?” Hunter said. “Will the figures be backdated? Also, what about cases that have a clinical diagnosis but negative lab tests, are they included in the confirmed cases or not?

“I have no problem with people using different case definitions but please be consistent or if you do change, run both in parallel for a few days so that no one believes the epidemic has suddenly got a lot worse. I suspect but can’t be certain that the underlying trend is still downwards.”

Facebook Twitter Pinterest Vietnamese olice wearing masks guard a road checkpoint outside Son Loi near Hanoi which is on lockdown due to coronavirus. Photograph: Yves Dam Van/AP

The rest of China is only reporting confirmed cases of Covid-19 infection. So are the 24 countries that have declared cases, which so far number 447 outside of China with two deaths – in the Philippines and Japan. “We are not seeing a dramatic increase outside of China,” said Ryan. The largest cluster was the 218 confirmed cases on the Japanese cruise ship Diamond Princess.

Nor, he said, are they seeing any significant shift in the pattern of mortality or severe illness. Those who were severely ill tended to be male and over 40 – and the more severe, the older they are, with the most dangerous cases in the 60s, 70s and 80s, Ryan said.

He suggested the best hope in the near future lay in the trials of existing antiviral drugs, which include some that are in use against Aids and HIV. Trials have started in China and it is hoped they will be extended to countries such as Singapore and Japan, both of which have significant numbers of cases. “You can imagine being a frontline clinician at the moment. Knowing which drug works would be a magical gift,” said Ryan.

The change in China to include what are known as “probable cases” appeared aimed at heading off complaints about the availability of tests and treatment for residents, as well as questions about whether officials have been underreporting cases.

The shortage of the testing kits has meant many sick residents have been unable to seek treatment, with hospital admission contingent on the test result. Health workers have been calling for authorities to broaden the parameters for diagnosing in order to treat more patients. Some have also questioned the reliability of the tests.

The crisis has also deepened in Hong Kong, where the education minister announced that schools would remain closed until at least the middle of March. They have been closed since the start of the lunar new year at the end of January.

In Vietnam, the authorities announced the lockdown of the commune of Son Loi, a farming region about 25 miles from Hanoi, for 20 days. Checkpoints were set up around the commune and health officials wearing protective suits sprayed disinfectant on vehicles.