World leaders and public health experts will focus on a variable called R0 in the coming months and years.
Pronounced 'R-nic', it stands for the estimated number of new infections caused by a single case.
In other words, if R0 is 2.5, one sick person is expected to infect 2.5 other people on average.
An R0 value below 1 indicates that the number of cases is decreasing, potentially allowing society to reopen. An R0 value above 1 means that the number of cases is increasing and quarantines or other measures may need to be reintroduced.
But the R0 is messier than it sounds. It relies on science, forensic investigations, complex mathematical models and often guesswork. It can vary greatly from place to place, from day to day, and fluctuate up and down due to local conditions and human behavior.
However, despite its volatility, R0 is expected to shape our world in the coming months and years as governments and health experts see it as the closest compass in the fight against the pandemic.
Here's a simple guide to how this indicator works, why it's important, and what to think about it.
What is R0?
This term, borrowed from demographic studies, is used to describe the birth rate.
R refers to reproduction and 0 to generation zero, such as patient zero. Together they are often referred to as the basic reproductive number.
It is calculated based on the inherent characteristics of the disease, such as how easily the disease is transmitted from one person to another, and human behavioral factors that affect the frequency of contact between infected and susceptible individuals.
The resulting numbers should help model the likely trajectory of the epidemic.
Assume that 1000 people have seasonal influenza and that the estimated R0 is 1.3.
It is expected to infect 1300 people. The second generation will infect another 1,690 people.
It can be added up. By generation 10, 42,621 people will get the flu in about 30 days.
However, epidemiologists emphasize that any R0 is only an estimate and is not perfect.
APaperA paper published last year in the academic journal Emerging Infectious Diseases called the indicator important, but warned that it "could easily be distorted, misunderstood and misused".
There is no consensus on how to measure it. Much of basic mathematics necessarily relies on guesswork and human factors that can change in unpredictable ways.
That's why most diseases spread, not numbers. SARS is often described as having an R0 of 2 to 5 - a big difference.
It is worth noting that scientists are still debating and revising disease estimates that have been studied for years. The R0 number for measles hasRange from 3.7 to 203.
Yet, for all its flaws, it is A useful abbreviation for experts studying the disease and managers trying to control it.
What is R0 for coronavirus?
There is actually no fixed R0. This number is best thought of as a starting point for virus behavior, without any real human or environmental factors.
New numbers are coming in all the time. Overall, however, studies now estimate the R0 for the pathogen that causes Covid-19 to be between 2 and 2.5.
This is much higher than the flu and lower than SARS, another coronavirus.
To understand how fast a virus spreads, you also need its serial interval, which is the average time between each successive infection. Some studies estimate that the coronavirus4 to 4.5 days. This is almost twice as fast as SARS and that is why the coronavirus is spreading so quickly.
However, the row spacing is considered to be more or less constant. People can significantly influence R0, which is why it attracts more attention than other indicators.
The term can also be used to describe a snapshot: an estimate of how the virus is proliferating on earth at a given time and place.
For example, at the beginning of the epidemic in ChinaResearch assessments, with a viral spread rate of 5.7 - a catastrophically high number.
Why is R0 so important?
Governments are increasingly using R0 as a measure of whether their country's cases are growing faster than they can control or are falling at will.
Interest in R0 has grown so much thatvideoIntroverted German Chancellor Angela Merkel, who is rarely associated with the viral video, explained that the variable has been viewed nearly 9 million times.
"The reproduction rate is currently about 1, so one person infects another," Merkel said at a press conference last week. "If we get to the point of infecting 1.1 people per person, our health system will be saturated by October."
When the number of patients in a country exceeds the number of beds in intensive care units, the death rate can skyrocket.
Mrs. Merkel added that if R0 is 1.2, Germany will cross that threshold in July. If R0 is 1.3, this will happen in June.
"Just look at how small the margins are," she said.
But real-time R0 estimates like in Germany,no matter how complicated it isvery speculative. This is an estimate based on other estimates, some more reasonable than others.
Nevertheless, it remains one of the few indicators by which we can gauge how effective quarantines and other policies are, and thus whether and when they are worth their enormous economic and social costs.
Does an R0 below 1 mean that the virus is defeated?
NO. Assuming the numbers are correct, this means that the spread of the virus has been halted.
When R0 is less than 1, it means that for every 100 sick people, less than 100 people are infected. Each generation will be less infected than the previous generation.
But people can still get sick, people can still die. It can take a long time for countries to fully recover from the virus, especially if the initial outbreak is severe.
For example, Italy recently estimated that its social and economic restrictions reduced the R0 of the virus to 0.8, a great achievement for a high price.
How long will it take for Italy to become like South Korea trying to reopenconfirmedAbout 10 new cases a day?
report from Italy15,918 new casesA handy shorthand for the number of people who may have been infected in the last five days. If R0 is 0.8, it would take 26.8 generations of the virus for new infections in Italy to catch up with South Korea.
Converted to four days, there are about 100 days per generation, which is the beginning of August. This can only be achieved if the status quo - lock-in - is maintained.
The next three and a half months of social restrictions will be expensive for the already strained Italian society and economy. But it is not guaranteed. Even South Korea has taken steps to reimpose quarantine if cases resurface.
Therefore, some experts believe that the goal of government action in the coming months and years is not simply to reduce R0 to the lowest possible level, but to keep it at an acceptable level.
In such a scheme, social and economic constraints would be lifted and reimposed to account for fluctuations in R0. But no one knew what true balance was, let alone how to achieve it.
AHarvard University researchIt is estimated that keeping new cases at a level that the health system can cope with could require adjustments to quarantine measures by mid-2022.
How does the government manage R0?
Official efforts to track R0 have been patchy but increasingly common.
Currently, some European countries report predictions of less than 1, but the degree of success remains uncertain.
Merkel eased restrictions after Germany set its own unemployment rate at 0.9, down from a recent low of 0.7. Governments face the painful dilemma of whether to view this increase as a necessary evil.
A French study estimates that a quarantine will eventually occurReduce R0 from 3.3 to 0.5an amazing transformation.
Still, the day after the study was published, the number of cases in France rose, in part because infections in nursing homes had not been reported before, a reminder that R0 estimates are not a perfect shorthand.
In India, one of the countries most concerned about the epidemic due to its large population and crumbling health system, the estimated R0 fell from 1.55 to 1.36. The number is low, but still above 1 - which means the number of cases is increasing.
It is difficult to assign a single R0 number to the United States. There are several simultaneous epidemics in the country, each with its own dynamics and at different stages of its life cycle.
It was founded by the former founder of InstagramWeb pageEstimate the value of Rt for each state, which is the change in R0 with respect to the data rate. Although it has nothing to do with professional epidemiologists or public health experts, it has generated widespread interest reflecting the desire for information about this useful but obscure indicator.
Max Fisheris an international reporter and columnist living in New York. He has reported on conflict, diplomacy, social change and more on five continents. he wroteTranslators, a column that explores the ideas and context of major world events. More about Max Fisher
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