The virus behind the COVID-19 pandemic succeeded where others failed because of a few super-spreader events, new research suggests.
Rather than having a reproduction number (the number of people each infected person goes on to infect) of between two and four - as the World Health Organization and others estimated early on - it was possibly as high as 11.4 in March and April, Polish scientists say.
They looked at the rate of expansion in the early months of the pandemic in China, the US and Europe, and ran it through computer models. They found the best explanation for how the virus spread was slow expansion at first - fooling many into thinking its reproduction number was low - before a "small population of super-spreaders" were infected.
"The simulations revealed two-phase dynamics, in which an initial phase of relatively slow epidemic progression diverts to a faster phase upon appearance of infectious super-spreaders," journal Royal Society Open Science, which published the research, said in a statement.
"In general [a] reproduction number calculated based on early epidemic development can be likely underestimated, and thus in the case of future epidemics must be considered with caution."
In late January - when the total number of people known to be infected was just 3000, 0.001 percent of the number today and almost entirely on the Chinese mainland - Harvard epidemiologist Eric Feigl-Ding reacted to a study which suggested the reproduction number of the virus was 3.8 with the all-caps phrase "HOLY MOTHER OF GOD", calling it "thermonuclear pandemic level bad... this [virus] is serious".
He was criticised at the time for using hyperbole.
"In the early phase, in which the doubling time (growth rate) is estimated based on individual case reports, the consequences of potential super-spreading events (such as football matches, carnival fests, demonstrations, masses or hospital-acquired infections) are negligible due to a low probability of such events when the number of infected individuals is low," the new study reads.
In other words, when only a few people are infected the chances of one of them attending an event with lots of uninfected people is low.
"In a given region or country, occurrence of first super-spreading events triggers transition to the faster exponential growth, in which subsequent super-spreading events become statistically significant and may become decisive drivers of the epidemic spread."
So once a single super-spreading event happens, the chances of them happening again increase - dramatically increasing the speed of the virus' spread.
Several countries managed to bring their local reproduction numbers down via lockdowns, cutting the virus off from potential new hosts.
Even without lockdowns, limiting the size of gatherings appears to have slowed local outbreaks - Spain and Italy both experienced "explosive" spread of the virus followed by "a phase of slower growth" once bans on large gatherings were introduced, before any complete lockdowns.
In early March there were fears of a super-spreader event in New Zealand after a man who had attended a concert by alternative metal band Tool tested positive for the virus. Director-General of Health Ashley Bloomfield said at the time the risk to other concert-goers was low.
At the time, it was not known the virus could be spread by people who were asymptomatic, nor that up to a third of people who contract the virus don't show symptoms at all.