What was the sequence of events leading to the pandemic?

The COVID-19 pandemic is the result of the rapid spread, on a worldwide scale, of the SARS-CoV-2 virus, which is responsible for COVID-19. By the 22^nd of October 2021, more than 242 million people had been infected by the virus and there were more than 5 million deaths. We are living through one of the largest pandemics in history. But how was SARS-CoV-2, which emerged in Wuhan in China at the end of 2019, able to spread throughout the world in only a few weeks? In this international study, scientists have established the unfolding of the key events and the responses of governments at the beginning of this health crisis.

The researchers established the timeline of the crisis between December 2019 and the end of March 2020 using data from scientific articles, the World Health Organization (WHO), and other experts in the field.

What was the sequence of events? The origins of the epidemic involved a jump in the species barrier, that is to say, the passage of a coronavirus from animal to human. The origin of the contamination remains unclear, although the hypothesis of a passage from bats (which act as a reservoir of coronavirus) to humans via an intermediate host, such as the pangolin, seems the most probable. Doctors at Wuhan hospitals began treating patients having an unidentified form of pneumonia in December 2019, with the 1st confirmed case of COVID-19 on December 8^th , 2019.

Later it began to be surmised that this virus was being transmitted from human to human, and that this process had probably already been underway since November 2019. This hypothesis took shape with the appearance of the first clusters of the disease. An initial internal note sent by municipal authorities to the city hospitals on December 30^th , 2019 was quickly leaked onto social networks.

The WHO then attempted to gather as much information as possible from the Chinese authorities. Some regions, including Taiwan and Hong Kong, put special measures in place, such as the monitoring of symptoms in passengers arriving from Wuhan. The first infected persons to be identified had all visited the city’s fish market, which closed for disinfection on January 1, 2020. China responded to the WHO on January 3^rd 2020, reporting 44 cases of pneumonia of unknown origin. On January 5^th, WHO notified all governments of this outbreak of unknown origin located in China. Two days later, Chinese scientists reported that they had isolated and sequenced the complete genome of the virus. Diagnostic PCR tests were developed and made available to hospitals in Wuhan on January 11^th.

During this period, possible cases of COVID-19 were detected in other countries, the 1^st in Thailand on the 8^th of January 2020, then in Japan on the 15^th, and in the United States on the 21^st. On the 22^nd of January 2020, the WHO reported 314 cases. The next day, they identified 581 cases, and the city of Wuhan was put under lockdown. But during this period, information on the illness was ambiguous and contradictory. No measures were recommended by the WHO at that time to limit international travel. A retrospective study concluded that before the end of January 2020, the majority of cases in China had not been detected, while the virus had already been able to spread widely across the country.

By the end of February 2020, more than 5 300 cases in 53 countries worldwide, with the exception of Africa, had been identified. At this time, the majority of countries had implemented only minimal health precautions. From the 11^th of March, the WHO began to use the word “pandemic”. By then, 118 000 cases had been reported in 114 countries. Italy became the 1^st country to announce a national lockdown, followed by Spain, then France on the 17^th of March 2020.By the 28^th of March, the number of infections in Italy and in the United States exceeded those in China.

The rapid actions taken against the epidemic, such as the sequencing of the virus genome, should be applauded. However, lessons must be learned from omissions or errors that occurred, in order to be able to prepare ourselves as thoroughly as possible for any future pandemic: alarm systems to identify emergent cases of an unknown illness need to be improved, as does monitoring of zoonoses (illnesses affecting animals) and, in particular, respiratory illnesses (which spread very rapidly). In today’s world, a highly transmissible pathogen can evolve very quickly towards a pandemic.