As Covid barrels through China, scientists around the world are looking for clues about an outbreak that could affect hundreds of millions of Chinese people and the global economy.
It is a big scientific guessing game to determine the size and severity of the surge in the world's most populous country because of the lack of credible information from the Chinese government.
Researchers pored over passenger data from five Beijing subway lines to determine the potential spread. A group of modelers tried to reverse engineer a government leak of case numbers from Chinese health officials. Scientists in Britain are coming up with their own estimates of vaccine effectiveness.
It is possible for any personal anecdote or social media report from China to be used in a model.
They are all trying to understand how fast the virus is spreading. There are a lot of people dying. China could be the source of a dangerous variant.
Scientists are bracing for the possibility of catastrophic outcomes as they sift through unreliable information. Estimates suggest that Covid could kill as many people in China in the next four months as it has Americans in the last three years.
Some countries are putting limits on Chinese travelers because they don't know what to do with them. The United States, Italy and Japan have said they will require a negative Covid test for people coming from China.
The policies reflect the limited visibility of the outbreak. Many people in China have little to no immunity to Omicron subvariants, which is why scientists' models point to an explosion and a high death rate. Their estimates are a lot different.
According to a study funded by the Chinese Center for Disease Control and Prevention, nearly a million people could die in the first few months after the end of China's "zero Covid" policy.
If China doesn't accept social-distancing mandates, as many as half a million deaths will occur by April and another million by the end of the year. There will be 1.7 million Covid deaths by the end of April, according to Airfinity, a British company.
The world had a pretty good idea of what was happening with the virus in China. The Communist Party published a low number of deaths and cases as a sign of its strict policy. A country wide system of lock downs, quark and mass testing kept the virus at bay.
The scientific community was largely unaware of the abandonment of zero Covid.
Siddharth Sridhar said nobody has a clue.
Predicting the path of the epidemic has been a challenge. In places like Britain, forecasts have been far off the mark. Scientists use reported Covid deaths as a reliable indicator of the potential size of an outbreak.
Data from the Chinese government is unreliable. China has only claimed 12 deaths from Covid. The country said it would only count those who died from respiratory failure, leaving out many who died because of Covid and other underlying diseases.
The speed of the spread suggests a higher number of deaths. Half a million cases were reported in one day. A million was reported by another person.
There are indications that officials are trying to keep respiratory deaths from being categorized as virus related.
A doctor at a private hospital in Beijing said that he and his colleagues found a note on a hospital desk that urged them not to write respiratory failure caused by Covid as the primary cause of death. The note was shared with a newspaper.
The doctor didn't know if the message was sent from the government or internal. Doctors in China have been warned not to write Covid on death certificates.
The scale of China's outbreak has been assessed using leaked information from government officials. According to data from national health officials, 250 million people had been exposed to the disease in the first 20 days of the year.
Some scientists said that it was possible that China had been suppressing data for months or that it was trying to make it seem like the outbreak had stopped.
Christopher Murray is the director of the Institute for Health Metrics and Evaluation at the University of Washington.
He thought that China was trying to make it look like the worst had passed. It is likely that China would have been able to fake the numbers for a long time.
Scientists are having a hard time assessing the about-face on China's messaging. China's state-controlled media warned about the dangers of the virus a month ago. The current Omicron variant is said to be mild and manageable.
Scientists and public health experts are worried that Omicron has looked less severe in other places due to the fact that the population in those places have a lot of immunity. Scientists are concerned that many more may die if China does not reimpose public health measures or increase vaccinations.
The Hong Kong researchers found that at least 250,000 lives could be saved by using social-distancing measures. Dr. Murray and his team found that social-distancing mandates could help spare hospitals from a concentrated surge of patients and reduce the death toll by 200,000 by April.
The Chinese public's perception of the outbreak will be important. Scientists said it could mean the difference between hospitals being able to treat their sickest patients or being completely overwhelmed.
The country's vaccine rate is a big variable. The booster rate for older Chinese people is lower than it is for the rest of the population. Three shots are important for Chinese vaccines that use inactivated virus, according to the World Health Organization.
James Trauer, an expert on modeling infectious diseases, said that people with previous shots should get extra protection within two weeks. Some places have extra time to get more people inoculated because the outbreak will not reach everyone at the same time.
The picture isn't clear as scientists study transportation patterns to understand how fast the outbreak could spread.
Scientists in Hong Kong analyzed passenger data from a few Beijing subway lines. Mobility in the city was thought to have dropped to low levels as people stayed home to protect themselves against the virus.
According to a senior fellow for global health at the Council on Foreign Relations, there are some indications that restaurants are getting more busy in big cities.
He said that it seemed to challenge the notion that people were exercising precautionaryly.
Many scientists rely on comparisons with Hong Kong because of the lack of better indications of how often Covid infections are fatal in China. The Chinese territory was vulnerable when Omicron began to spread there.
In the early stages of China's outbreak, some models assumed that it would have the same death toll as Hong Kong's. Thousands of people died within months of Omicron spreading in the area. The toll in China would be much higher.
There are significant differences. Hong Kong had better vaccine coverage than China at the beginning of its surge.
China's population-wide vaccination drive was earlier than in Hong Kong, which meant the effects of inoculations had more time to wear off. Hong Kong and China both gave the option of Western vaccines with newer technology. Hospitals might have a harder time handling the surge in some areas.
Concerns have been raised that the size of the outbreak could allow imported versions of Omicron to evolve into a more dangerous version.
Scientists are skeptical of such a scenario.
The United States has more Omicron subvariants that are more infectious than those that China has reported. After Italy mandated testing for travelers from China, it said the first cases it was able to sequence were caused by an Omicron variant in Italy. The European Union health officials said that the screening of travelers from China was not justified.
James Wood, an infectious disease expert at the University of New South Wales in Australia, estimated that most people around the world had caught the disease. There have been more infections in China than anywhere else.
The additional reporting and research was done by several people.