From (answers to) a related question, the WHO had mostly changed their public stance on the human-to-human transmission of COVID-19 after Jan 22, while giving mostly "negative signals" on this matter of human-to-human transmission before Jan 22.

Interestingly, on Jan 17, the European Centre for Disease Prevention and Control (ECDC) put out a rapid report saying that:

The occurrence of a few cases having no history of contact with the implicated market or other any similar market suggests the possibility of the infection source being more widely distributed, or of human-to-human transmission similar to other coronaviruses such as SARS-CoV and MERS-CoV. The occurrence of a few family clusters also supports this latter hypothesis. However, there is currently no knowledge on the transmission mode or risk factors for transmission. Furthermore, there is no information on the disease spectrum or risk factors for severity. In light of these unknowns, we cannot exclude the possibility of low-level virus circulation in the Wuhan community and thus the possibility of other clusters occurring in the near future. [...]

  • There is a high likelihood of case importation in countries with the greatest volume of people travelling to and from Wuhan (i.e. countries in Asia).
  • There is a low likelihood of importation of cases in EU/EEA countries, due to the less extensive traffic of people with Wuhan.

For comparison, what did the US CDC say on this matter (human-to-human transmission) within this period (i.e between Jan 14 and 21), and did they also asses back then (like the ECDC did) the likelihood of importation into the US?

Insofar I was able to find, by Jan 31, the US CDC was clearly convinced of human-to-human transmission:

On January 31, 2020, CDC published updated PUI guidance (8) in response to the evolving global epidemiology of 2019-nCoV, including the rapid geographic expansion and documentation of person-to-person transmission (9). Updated guidance emphasizes 2019-nCoV testing for symptomatic persons in close contact with patients with laboratory-confirmed 2019-nCoV infection, persons returning from Hubei province in addition to Wuhan City, and persons from mainland China requiring hospitalization because of fever and lower respiratory tract illness. Additional refinements to this approach likely will be needed in the future as understanding of 2019-nCoV epidemiology continues to improve.

But what did they say roughly two weeks earlier, before the WHO had confirmed human-to-human transmission?

  • 1
    Shouldn't you elaborate the definition of your term "human to human transmission"? If there was no "human to human transmission", how could the virus spread to many people in Wuhan? before January? Flying in air? – Kentaro Apr 9 at 14:20
  • @Kentaro: it's not my term. It's a medical/technical term used in those reports, and actually appears in the quotes I've included. (The CDC calls it "person-to-person transmission" unlike the ECDC which used "human-to-human transmission" literally.) If you don't know what those terms mean, ask on med SE. – Fizz Apr 9 at 14:29
  • you are correct about it, right. But there is nothing other than "human to human infection" that spreads virus. – Kentaro Apr 9 at 14:35

On January 17th, the CDC announced that COVID-19 screening would begin at "three U.S. airports that receive most of the travelers from Wuhan, China: San Francisco (SFO), New York (JFK), and Los Angeles (LAX) airports."

They also state in this press release that:

CDC is actively monitoring this situation for pertinent information about the source of outbreak, and risk for further spread through person-to-person or animal-to-person transmission. CDC may adjust screening procedures and other response activities as this outbreak investigation continues and more is learned about the newly emerging virus. Entry screening alone is not a guarantee against the possible importation of this new virus but is an important public health tool during periods of uncertainty and part of a multilayered government response strategy. As new information emerges, CDC will reassess entry screening measures and could scale activities up or down accordingly.

This indicates that an assessment of the likelihood of importation of the virus into the US was completed, and a decision was made that only the airports with the largest amount of traffic needed to be screened. They do however note that this is not a guarantee that the virus will not be imported. Further on in the briefing, they mention that:

China health officials report that most of the patients infected with 2019-nCoV have had exposure to a large market where live animals were present, suggesting this is a novel virus that has jumped the species barrier to infect people. Chinese authorities additionally report that several hundred health care workers caring for outbreak patients are being monitored and no spread of this virus from patients to health care workers has been seen. They report no sustained spread of this virus in the community, however there are indications that some limited person-to-person spread may have occurred. CDC is responding to this outbreak out of an abundance of caution, ready to detect people infected with 2019-CoV.

This briefing suggests that while the CDC was preparing for human-to-human transmission to be confirmed, and may have even thought that this was likely, it did not have the data to confirm this.

Later on, on the 21st, the CDC released another briefing after the first COVID-19 positive patient was identified in the US. The briefing notes that the patient returned from China on the 15th, before the screening mentioned above was implemented. This release, however also does not state that human-to-human transmission is confirmed, but mentions that

While originally thought to be spreading from animal-to-person, there are growing indications that limited person-to-person spread is happening. It’s unclear how easily this virus is spreading between people.

At this point the CDC also implemented screening at two more airports, Atlanta and Chicago.

It seems clear, therefore, that the CDC did not have access to data which confirmed human-to-human transmission, although the steps taken to implement health screening at airports, and the advice given to healthcare providers on January 8th indicates that it certainly thought that the likelihood of this was great. Clearly the health screening also proves that spread from Wuhan was seen as a possibility, although we can infer from the fact that this was initially only set up at three airports that this was either not seen as a large possibility, or that the spread via air-travel was not expected to be high enough to warrant a complete lock-down.

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To back up a bit, the Jan 8 CDC notice recommended isolating people who had traveled to Wuhan

Although the etiology and transmissibility have yet to be determined, and to date, no human-to-human transmission has been reported and no health care providers have been reported ill, CDC currently recommends a cautious approach to symptomatic patients with a history of travel to Wuhan City. Such patients should be asked to wear a surgical mask as soon as they are identified and be evaluated in a private room with the door closed. Personnel entering the room to evaluate the patient should use contact precautions and wear an N95 disposable facepiece respirator. For patients admitted for inpatient care, contact and airborne isolation precautions, in addition to standard precautions, are recommended until further information becomes available.

The Jan 17 advisory notes a growing connection between human-to-human transmission

Chinese authorities additionally report that they are monitoring several hundred healthcare workers who are caring for outbreak patients; no spread of this virus from patients to healthcare personnel has been reported to date. Chinese authorities are reporting no ongoing spread of this virus in the community, but they cannot rule out that some limited person-to-person spread may be occurring.


Thailand and Japan have confirmed additional cases of 2019-nCoV in travelers from Wuhan, China. It is possible that more cases will be identified in the coming days. This is an ongoing investigation and given previous experience with MERS-CoV and SARS-CoV, it is possible that person-person spread may occur. There is much more to learn about the transmissibility, severity, and other features associated with 2019-nCoV as the investigations in China, Thailand, and Japan continue. Additional information about this novel virus is needed to better inform population risk.

The Feb 1 update states it outright (emphasis mine)

Early in the outbreak, many of the patients with respiratory illness caused by 2019-nCoV in China had exposure to a large seafood and live animal market, suggesting animal-to-human transmission. More recently, cases have been confirmed with no exposure to animal markets, indicating that person-to-person spread of the virus has occurred. Chinese officials report that sustained person-to-person spread in the community is occurring in China.

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