Undetected SARS-CoV-2 community transmission drove the first wave of COVID-19

The beginning of the coronavirus disease 2019 (COVID-19) pandemic was officially announced on March 11, 2020 by the World Health Organization (WHO). The appearance of some unusual cases of pneumonia back in December 2019 had been reported from Wuhan, China, followed by reported cases in the United States and Europe the next month on January 21 and 24, 2021, respectively.

Recently, researchers find that the exponential spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for COVID-19, occurred during this period but went largely unrecognized. International travel was the major stimulus for the numerous introductions that occurred at this time, coupled with poor case detection strategies that left almost 97% of cases undetected.

Study: Cryptic Transmission Of SARS-Cov-2 And the First COVID-19 Wave. Image Credit: Alones / Shutterstock.com


The introduction of SARS-CoV-2 into a number of countries in Europe and Asia, as well as into the United States over the weeks following its emergence in Wuhan was reported early in January 2021. Only a few daily cases were detected at this time, most of which were the result of testing individuals who exhibited COVID-19 symptoms and had a history of travel to China.

However, many earlier papers have shown that SARS-CoV-2 entered these countries much earlier than it originally believed, which indicates that the testing and screening policies used at the time were inefficient. The current study used the Global Epidemic and Mobility Model (GLEAM) to explore the global spread of SARS-CoV-2.

This model links the various possible pathways of virus spread using data from the earliest days of the pandemic, to reveal the potential transmission of SARS-CoV-2 during this cryptic phase. The researchers included data from the U.S. and Europe, incorporating human mobility real-time, and population data using mechanistic modeling with changes in the patterns of contact and of people movement to fit the non-pharmaceutical interventions (NPIs) in each region.

Study findings

The researchers found that the median number of new cases occurring daily in the U.S. and Europe were much higher than official numbers, thereby indicating that many transmission events had occurred before rigorous testing and surveillance strategies were put in place without the consideration of travel history. In other words, significant community transmission had occurred already while travel history was considered a risk factor.

Initially, only a tenth of cases was being detected by March 8, 2020, in the U.S. and 3.5% in Europe. With increased testing capacity, the model estimates that about twice the original number was being detected. As early as February 21, 2020, some areas had already experienced outbreaks.

Defining local transmission for a state or country as the date on which ten or more new infections were first recorded, the model demonstrates that by the week of January 26, 2020, and February 2, 2020, California and New York might have already become centers of local spread. The model describes the spread of SARS-CoV-2 over the U.S. and Europe largely in accordance with the data from the John Hopkins University Coronavirus Resource Center.

The origin of the introduction of SARS-CoV-2 to the U.S. and Europe owes little to China, according to this model, while local spread becomes far more important. The latter was, of course, sparked by an initial spurt of introductions from abroad, but gave way to cryptic transmission sustained by the internal circulation of the virus.

Thus, Texas, California, and Massachusetts saw 70% to 80% of introductions owing to domestic introductions, while in Europe, 60-70% of the introductions were from within the country. Once a local spread chain has been initiated, additional chains can still begin due to further introductions of the virus.

In other words, a seeding introduction may have brought SARS-CoV-2 from China to the U.S. and Europe in January 2020; however, later introductions led to additional introductions and onward transmission. Air travel was shown to be the major driver of the seeding introduction and early spread.

However, regions with high population density would be prone to increased contacts before NPIs were implemented. This effect was not separated from the effect of increased seeding because of high travel volumes linked to similar population sizes.

The weekly deaths, as estimated by the model, matched well with the reported values from selected states and countries. This was evident in Belgium, with the highest estimate of 13% infection attack rate compared with the highest reported mortality at 8.5 per 10,000 individuals by July 4, 2020. A similar agreement was seen with the U.S. northeastern states.


The findings of this study show uncanny agreement with the reported figures from the cryptic phase of the pandemic, thus allowing researchers to understand the mechanism by which the pandemic evolved throughout the world. Over 70% of early introductions into Italy, for instance, were from China, thereby indicating the role played by imported infections at this point.

Further, most importations into the U.S. came from Europe through April 2020, with less than 1% from China, mostly at a very early or seeding point of the outbreak there.

Over time, SARS-CoV-2 was introduced through multiple routes that saw rapid changes, most of which were responded to in a reactive fashion. This resulted in travel restrictions on countries where local outbreaks had already been confirmed. However, with an earlier extension on testing without the requirement for a travel history to a place with high transmission rates such as China, cases could have been detected earlier and transmission chains halted.

The current model also sheds light on the effect of lockdowns, showing that most COVID-19 diagnoses happened after local spread had already begun. The earlier the NPI initiation date, the smaller the peak of the first wave of COVID-19 in that place, confirming that NPI timing is key to keeping the case burden low.

Overall, our results strengthen the case for preparedness plans with broader indication for testing that are able to detect local transmission earlier.”

The researchers used modeling and analytic methods that allowed them to map the spread of SARS-CoV-2, as well as other emerging viruses. These approaches can be applied to evaluate the risk of spread in real-time, thus helping to predict the cryptic transmission phase. As such, these methods could be useful in planning public health strategies to regulate international travel, as well as assessing the threats of emerging variants in low-income regions that have limited resources for testing and surveillance.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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