COVID-19 twice as contagious as previously thought – CDC study

A person with COVID-19 could infect up to 6 others – not 2 or 3 as previously thought – making measures to control the pandemic ever more critical, according to a new CDC study.

Los Alamos National Laboratory in New Mexico, US studied data from Wuhan, China and estimated the basic reproductive number (R0) of SARS-CoV-2, the coronavirus that causes COVID-19, to be 5.7[1]

This is double the previously estimated R0 of 2.2 to 2.7, meaning that the COVID-19 pandemic may be much more difficult to contain.

“Results show that quarantine and contact tracing of symptomatic persons can be effective when the fraction of unidentified persons is low,” authors Steven Sanche, Yen Ting Lin, Chonggang Xu, Ethan Romero-Severson, Nick Hengartner, and Ruian Ke write. “However, when 20% of transmission is driven by unidentified infected persons, high levels of social distancing efforts will be needed to contain the virus (Figure 6), highlighting the importance of early and effective surveillance, contact tracing, and quarantine.”

“We need to now be even more vigilant than previously thought given the higher revised R0,” epidemiologist Eric Feigl-Ding said.

He points out that this R0 is much higher than the recent Imperial College review of published R0 of 3.87[2]

“Ergo, a 5.7 is on a considerably different level of infectiousness,” Dr Feigl-Ding said.

Dr Feigl-Ding explains that R0 is the “R reproductive number at time 0 before countermeasures”.

He points out that this is not the R(effective) at current time under mitigation measures such as distancing and testing, tracing and quarantine, which are expected to slash chains of transmission.

According to the study’s authors point out that at least “82% of the population has to be immune, through either vaccination or prior infection, to achieve herd immunity to stop transmission.”

The authors of the study issue a stark warning to policy makers.

“Our results suggest that a combination of control measures, including early and active surveillance, quarantine, and especially strong social distancing efforts, are needed to slow down or stop the spread of the virus,” they write. “If these measures are not implemented early and strongly, the virus has the potential to spread rapidly and infect a large fraction of the population, overwhelming healthcare systems.”

But all hope is not lost, according to the researchers.

“Fortunately, the decline in newly confirmed cases in China and South Korea in March 2020 and the stably low incidences in Taiwan, Hong Kong, and Singapore strongly suggest that the spread of the virus can be contained with early and appropriate measures,” the Los Alamos scientists conclude.

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8 Responses to COVID-19 twice as contagious as previously thought – CDC study

  1. teddy clifton says:

    This makes absolutely no sense to me. Surely it becomes largely dependent on the number of exposures to others.If one is in isolation and is infected the infection transmission could be zero. Whereas a person infected in a crowded room could infect hundreds.The Italians learned that by placing infected people in with people in hospitals that weren’t infected.

    • Anonymous says:


    • S Jones says:

      It’s a statistical measure so, of course, individual conditions will vary.

      Think of it this way: If you put 20 people in a locked room w/ one unsuspected carrier, how many people would be infected within a couple of hours. Knowing that about different diseases helps assess broader risk.

      Of course, if all these people wore protective gear, avoided touching and coughed into their arms, or whatever, you’d dampen the contagion but R0 is instead the natural, unsuspected transmission rate. It’s a baseline to compare to other diseases. For example, Ebola and sexually transmitted diseases are bad but considered to be weakly contagious in the “locked room” sense because they require direct contact with bodily fluids which strangers tend to avoid.

      The reason COVID is a pandemic is because it has a higher R0 (it spreads pretty easily) AND carriers can be asymptomatic (i.e. don’t advertise their sickness) for some time while they can infect other people.

  2. John P says:

    @Teddy, you are correct that the number of people someone infects depends on how many people they contact with. We have seen many instances where an contagious person has attended an event and infected scores of people. The R0 value is a type of average–across the board, how many people a typical person infects, taking into account all the behaviors in which people engage and all the environments in which they might live.

    R0 is more useful for mathematical modeling than for actually predicting how many people an individual will infect, since so many variables are at play. You can think of it as more of a contagiousness index. The higher the R0, the faster the disease will spread. The H1N1 influenza from 2009 had an R0 of ~1.47, so it was less contagious than SARS-CoV-2. Measles has an R0 of 12 or more, so it is far more contagious than the coronavirus.

  3. d says:

    HELLO just connect the dots already – with an R0 value at 5.7 this thing started to cover the world the moment its particular mutation started spreading from human to human. There’s no way the initial test-chasing of this virus can paint the bigger picture needed to tackle it correctly. Just as with Swineflu 2009.

    I don’t buy Korea’s emphasis on testing/tracing at all – it’s the masks! Look at Japan, even regular flu cases started to go down as soon as the news of Wuhan broke and mask wearing went up(mask wearing later went down and then Japan’s critical cases went up again). You really think a 5.7 partly airborne virus with this many asymptomatic carriers wouldn’t make its way to Japan instantly?

    And holy sh** do I have to explain everything?? Did anyone ever consider the amount of saliva pollution to be the main reason for complicated cases? Just as what you are seeing happening in Hospital-environments with young doctors getting serious infections. New York subway anyone? You’re going to see more serious cases earlier in connected populated areas.. especially where elderly are mixing socially; Italy, Spain etc… for ex. not so much Germany and Sweden – different cultures – unless we are talking immigrants and more social age-mixing sub-cultures in these countries or care home-leaks.

    This thing is way, way less lethal than you think. That’s the way to solve this puzzle just with simple thought-experiments. The critical cases were never the actual spread and people have probably been dying from this for many months now outside of China, disguised as the seasonal flu. The spread is just waiting to bubble up again after these lockdowns and you can’t chase it.. focus on realistic measures. 

    One of the first decent mass testings in the hard hit German municipality of Gangelt shows estimation of death rate to be around 0.37%. And if the initial dose of infection matters it might be even less in more rural areas. Also, if indoor pollution is key then I wonder if maybe the warmer countries with outdoor shops/markets might have a better chance at this..  I hope… but I wouldn’t count on it.

    So then why are we tanking our economies and risking future misery ? Lets not! Solution: end lockdowns, boot up societies again.. keep social distancing, enforce DIY/simple-mask wearing in indoor shared spaces if needed, further protect the risk-groups, and ease up restrictions periodically until we reach treatment, vaccine or herd-immunity. Antibody pass? Sure, If you work with healthcare, elderly etc. Want a decent non-overkill take on the pandemic? Look at Sweden. Add masks to that and you got yourself a winner… just like slow-on-action-Japan, except you would actually be aware of the effect of your decisions.

    I’m not a scientist, but I am clever and defiant in my thinking – and I’ve been painfully right about everything for over a month now… but who would listen to me? GEEZ! I know there are clever timid scientists out there connecting the dots. SPEAK UP!

    • d says:


      Been off on a big break from Covid-19 digging so finally did end up finding some names of individuals hip to the thinking that are good to take a look at regarding the actual figures. Actually quite a lot of decent videos being recommended on youtube once I found these. Enjoy getting closer to the truth of this fiasco.

      Dr John Ioannidis – Stanford
      Jay Bhattacharya – Stanford

      Also heard a very interesting rant/take on allowing the virus to run its course.. letting it find its hosts so It wont have to mutate into anything more lethal down the line – I know sh** about this ofc and I would take this guy with a grain of salt ..its compelling though

  4. Stephen M Colby says:

    R is a function of biology and social mixing context. So there should be a mixing factor M in the model, and an M0 to go with R0. And,the R0 is a function of M0. And the M0 for Wuhan is not the same as for other areas. Note Sweden.

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