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How early did local transmission of the SARS-CoV-2 virus begin in the United States? For the second time this week, scientists have proposed a new estimate. This one, from scientists at the Centers for Disease Control and Prevention, suggests that transmission likely began in late January or early February on the West Coast and that the virus spread undetected for more than a month.

Scientists from the agency, and research groups they collaborate with, concluded community transmission in the U.S. began with a single unidentified imported case from China.

Kristian Andersen, an expert in viral genomes from Scripps Research, said the new study underscores that early chains of transmission were missed in the United States, which as a consequence lost an opportunity to prevent spread of the virus from taking root here.


The CDC team drew on multiple streams of data, including retrospective testing of nearly 11,000 samples collected through CDC-led influenza surveillance networks.

If Covid-19 cases were being mistaken for flu in the early days of the outbreak, one would have expected to have seen some positive tests among those samples. But the first that was positive was collected from a patient in Washington state on Feb. 25 — two days before authorities in California reported finding an infection in a woman who had not traveled outside the country and who was not a known contact of people who returned to the U.S. infected with the virus.


CDC Director Robert Redfield pointed to the work as evidence that the agency — which has been savaged for the early failure of a Covid-19 test that left the country with limited capacity to look for the virus in the early weeks of the outbreak — had always been alert to the virus’ movements in the United States.

“We were never blind when it came to surveillance for Coronavirus 19,” he told reporters during a briefing on the new findings, published Friday in the CDC’s online journal Morbidity and Mortality Weekly Review. “The reality is the surveillance systems that CDC had developed over the years for respiratory viral diseases, particularly influenza-like illness, really did give us eyes on this disease as it began to emerge.”

Redfield’s statement seemed to ignore the fact that the flu surveillance samples were not tested for Covid-19 in real time, when they might have helped public health departments track and halt spread of the new virus.

“That’s a preposterous statement,” Andersen told STAT. “It’s a sad fact that the United States missed the boat on getting adequate testing set up early enough to be able to stop the virus in its tracks — it’s likely one of several reasons we have by far the most cases of any country in the world.”

As of Friday, the U.S. has recorded more than 102,000 deaths — 28% of the global total.

The CDC findings challenge to a degree those released earlier this week in a preprint article from Michael Worobey, a professor of evolutionary biology at the University of Arizona. Preprint articles have not yet undergone the normal peer-review process at a scientific journal.

Worobey and colleagues argued that the person who started the first chain of transmission to take root in the country — in Washington state — entered the U.S. around mid-February, either from Asia directly, or via British Columbia, Canada.

Their estimate, based on analysis of genetic sequences of viruses retrieved from infected people, in turn challenged work from Trevor Bedford, a computational biologist at Fred Hutchinson Cancer Research Institute, that suggested the Washington state outbreak began with the first Covid-19 case diagnosed in the country, a man who returned from China on Jan. 15.

But the CDC scientists came to a different conclusion after analyzing data from four sources: genetic sequences; retrospective testing of stored influenza surveillance samples; a review of emergency room records from counties that had early cases of Covid-19; and the retrospective discovery of two people in California who died from Covid-19 in early and mid-January, and an infected cruise ship crew member who departed from San Francisco on Feb. 11. Public health officials only later realized those individuals had been infected with the virus.

The combination of those data sources led the CDC team to conclude community transmission in the U.S. began in late January or early February. Worobey estimated local transmission began in the U.S. in Washington state around Feb. 13, though it could have been as early as Feb. 7 or as late as Feb. 19.

“The take-home message from the northern California thing seems to be that yes, there definitely were early cases and some early community transmission, but when you just fast forward a little bit in time, none of it seems to have established itself into a sustained transmission cluster like you see in Washington or later in New York,” Worobey told STAT.

One of the authors of the CDC study, Greg Armstrong, said the agency’s scientists disagreed with how firm the two earlier estimates were in assessing when transmission began in the United States. “We thought there was a considerable amount of uncertainty still, given how few data were available,” said Armstrong, who is CDC’s deputy incident manager for the response.

The estimates of when the virus actually started transmitting in the U.S. could change again, Armstrong said, noting pathologists continue to investigate suspicious deaths from early in the year and to send samples to the CDC for analysis. “It is quite possible we will find additional cases in the future,” he said.

  • We had this thing in Washington in November. My family is in King County, I volunteer weekly at my childrens elementary school. The kids, the teachers, the office and support staff, then then temporary staff…roughly half of 480 school members… were out for weeks on end from 1st week of November 2019 through mid/late January – and it rotated, then repeated, depending on immunity of the individual. My kids had 104-105 fevers, chest pain, headaches, felt like they couldn’t think, lethargic, couldn’t eat, wouldn’t drink… laid on the floor for days, because the hard wood was more cool than their beds. Pediatricians office said palliative care, so I did, for a long couple weeks – kids mucinex, tylenol, ibuprofen, gatorade, broth, popsicles, and prayers. We’ve had a lot of respiratory illnesses under our belts over the years, and half the house being immuno suppressed due to one thing or another however, whatever it was that we had, that we tested NEGATIVE FOR FLU for … was the wickedest crap ever. And now that I’ve seen and heard about cases local that were tested and confirmed covid 19 cases I know, without a shadow, without a test, the neighborhood had it already, so we’re in a bubble here. In my town, because it already got us. Funny thing, not even a sniffle in the area since February. No one was out of school sick, no one has been looking or acting run down or lethargic, no one has had anything but allergies. We still can’t get covid tests here either. I look forward to an antibody test, just to get out of the mix.
    Be safe out there folks, <3

    • B. Menendez: I find your history of an illness in your town very interesting and think there was something far out of the ordinary going on, but it does not really sound like the Wuhan virus because not all the symptoms fit.
      I once hitchhiked from California to Dallas (gave up and took the bus from there) and went through Lake Tahoe on the way -a couple months after getting back east, I got very sick for months – just being very, very weak was the main symptom, though some definite mental fog too, very vague and doctors blew me off – eventually I got better, came back to California and read in the SF paper about people in Lake Tahoe having exactly same symptoms, doctors debating if it was mass hysteria – – but the thing was, I never knew about this illness in that area until AFTER I got back and was mostly recovered.

      We do not really know how often zoonotic jumps occur, but then deadend (Thank God!) and infect a few people or a few hundred (like Tahoe) without going further.

      But if the disease in your area WAS the Wuhan virus, the symptoms were not typical, so can you go through all the symptoms again? Because we need to know if it acts differently from we have been told so far.

  • -But no body tested any flu samples- so that would be the “CONFIRMED approx. date”- We live east Lane co. mts, Oregon , an hour away from civilization, hosp./Dr., and EMT’s were heading back from previous call.
    I personally know of an infant (1 yr.old) that had super high temp, respiratory, and was having seizures ,he died twice before getting to hospital. His mother had to pull over and perform c.p.r. and he died after they switched him to ambulance that met them.
    Sorry so long- the point- when he got to hospital he was diagnosed w/ 3 different virus- one was SARS. and that was in end of Novenber-beginning of December. That should test the influenza test- we still don’t have tests for regular people not on front line and not in hospital out here. ( and baby li ed -scary 3 moths)

  • I agree that having had early testing would have been beneficial, but it is not at all clear that it would have quenched the spread of the disease. What testing has to do is to find the asymptomatic person; and the only way to find them is by wide scale random, unbiased sampling. Even at the current testing rate of 400,000/day or 2,000,000/five days, that would cover less than 20% of NYC, resulting in finding less than 20% of the asymptomatic persons. That would likely have had an important impact upon the spread of the disease in NYC, if we kept doing that every week. But could we really have expected that we could have come up to that rate of testing so quickly and concentrated all our efforts in one small geographical area?

    • I am not sure you need to give out a very large number of tests randomly for contact tracing to help. I think you are going to try to ask a recently infected person who he has had contact with, and then test those people. You need far fewer tests to do that.

    • LARRY – South Korea did much better than the US and used contact tracing well -but all of Asia wears face masks routinely – maybe that made more difference than all the rest of it combined. It is looking that way now.

  • So as a medical provider in SoCal we aren’t allowed to discuss it. We had this damn thing last winter. Period. If you sample size is 30 million under then your morbidity and mortality is closer to H1 N1. Shame on the media for promoting ‘complication of COVID” instead of pneumonia, morbid obesity, type two, type one diabetes, various levels of immunosuppression from polypharmacy.
    Combine that with an electorate that likes the fast food, doesn’t exercise, oh and now thinks that science is full of crap well then I guess you have a pandemic for the first time in the United States since polio. We had this damn thing through Southern California months ago we all F**king know it. Colleagues with vblogs from their practices concur. Their videos removed in an hour.
    Agree bad flu year. Shut down $1 trillion economy for what? The number of people who will die from poverty, job loss, will be so much greater.

    • I am not trying to fight with anyone but I am interested in finding out about any kind of censorship about the virus. Just to let you know first, I believe there is some ongoing censorship which is intended to prevent the public from blaming China for the pandemic. We are now being told the Wuhan wet market was not the source of the virus – it is hard for me to imagine the Chinese authorities did not take samples of animals there, and, if they had all been found negative, would not have told us that at the time. Without seeing all the communications from the government in a timeline, it’s very hard to get a grasp of it, but I see, repeated as fact, the claim the government blamed the wet market, but now there are claims it was not the wet market. And we see major media in the US saying the theory it escaped from the virology institute is a “conspiracy theory” but they do not provide real evidence of it – for that matter, we see the woman who ran that place supposedly making videos saying she is fine – therefore, has not been murdered in some secret prison for political opponents – but she has a mask on the entire video.
      Any information you have on videos and blogs being interfered with I am very interested in seeing. Thanks in advance.

    • Analysis of excess deaths in numerous cities/countries suggest this is not just a bad flu season. What has already occurred in places like Iran, Northern Italy, Spain, NYC, Brazil, etc should make painfully obvious how serious this virus is. I do agree with you that it is a shame that so many Americans are unhealthy and put themselves at higher risk. And the main reason we are shutting down is to help the medical providers from being completely overwhelmed. It’s pretty sad to see a medical provider that doesn’t understand that.

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