Undetected community transmission of SARS-CoV-2 led to first wave of COVID-19


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

Recently, researchers found that the exponential spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the virus responsible for COVID-19, occurred during this period but has remained largely unrecognized. International travel was the main driver of the many introductions that took place during this time, coupled with poor case detection strategies that left almost 97% of cases undetected.

To study: Cryptic transmission of SARS-Cov-2 and the first wave of COVID-19. Image Credit: Alone / Shutterstock.com


The introduction of SARS-CoV-2 in a number of countries in Europe and Asia, as well as the United States in the weeks following its emergence in Wuhan was reported in early January 2021. Only a few Daily cases were detected around this time, most of which were the result of tests on people who showed symptoms of COVID-19 and had previously traveled to China.

However, numerous previous articles have shown that SARS-CoV-2 entered these countries much earlier than originally believed, indicating that the testing and testing policies used at the time were ineffective. The current study used the Global Outbreak and Mobility Model (GLEAM) to explore the global spread of SARS-CoV-2.

This model links the different possible routes of virus spread using data from the early days of the pandemic, to reveal the potential transmission of SARS-CoV-2 during this cryptic phase. The researchers included data from the United States and Europe, integrating real-time human mobility, and population data using mechanistic modeling with changes in the contact and movement patterns of people to adapt to changes. non-pharmaceutical interventions (NPI) in each region.

Study results

The researchers found that the median number of new cases occurring daily in the United States and Europe was much higher than official figures, indicating that many transmission events had occurred before testing and surveillance strategies. rigorous measures are put in place regardless of travel history. In other words, significant community transmission had already occurred when travel history was considered a risk factor.

Initially, only a tenth of cases were detected as of March 8, 2020, in the United States and 3.5% in Europe. With increased testing capacity, the model estimates that about double the original number was detected. As of February 21, 2020, some areas had already experienced epidemics.

Defining local transmission for a state or country as the date that 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 may already have become centers of local distribution. The model describes the spread of SARS-CoV-2 in the United States and Europe largely according to data from the John Hopkins University Coronavirus Resource Center.

The origin of the introduction of SARS-CoV-2 in the United States and Europe owes little to China, according to this model, while the local spread becomes much more important. The latter was of course triggered by a first wave of introductions from abroad, but gave way to cryptic transmission maintained by the internal circulation of the virus.

For example, Texas, California and Massachusetts saw 70-80% of introductions due to domestic introductions, while in Europe, 60-70% of introductions came from within the country. Once a chain of local spread has been initiated, additional chains may still start due to further introductions of the virus.

In other words, a seeding introduction may have brought SARS-CoV-2 from China to the United States and Europe in January 2020; however, subsequent introductions led to additional introductions and subsequent transmission. Air transport proved to be the main driver of seedling introduction and early propagation.

However, densely populated areas would be subject to increased contact prior to the implementation of NIPs. This effect was not separated from the increased seeding effect due to the high displacement volumes associated with similar population sizes.

Weekly deaths, as estimated by the model, corresponded well to values ​​reported by some states and countries. This was evident in Belgium, with the highest estimate of the infection attack rate of 13% compared to the highest reported mortality at 8.5 per 10,000 individuals as of July 4, 2020. A similar agreement was reached. been observed with the northeastern United States.


The results of this study show a strange agreement with reported figures from the cryptic phase of the pandemic, allowing researchers to understand the mechanism by which the pandemic evolved across the world. More than 70% of early introductions to Italy, for example, came from China, indicating the role played by imported infections at this stage.

In addition, most imports to the United States were from Europe until April 2020, with less than 1% from China, mostly at a very early stage or at the starting point of the epidemic there.

Over time, SARS-CoV-2 has been introduced through multiple pathways that have undergone rapid changes, most of which have been dealt with reactively. This resulted in travel restrictions in countries where local outbreaks had previously been confirmed. However, with earlier expansion of testing without the requirement of travel history to a place with high transmission rates like China, cases could have been detected earlier and chains of transmission interrupted.

The current model also sheds light on the effect of blockages, showing that most COVID-19 diagnoses have occurred after local spread has started. The earlier the launch date of the NPI, the smaller the peak of the first wave of COVID-19 there, confirming that the timing of the NPI is critical to keeping the burden of cases low.

Overall, our results strengthen the case for readiness plans with a broader indication for tests capable of detecting local transmission earlier.. “

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

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