This Is When 50 Percent of COVID Infections Happen, CDC Says

The CDC now says that half of coronavirus transmission occurs before the onset of symptoms.

Months into the coronavirus pandemic, experts are still trying to pinpoint when the bulk of transmission occurs. Part of the reason why COVID-19 numbers increase exponentially when they do is because people can and do infect others without being aware that they are infected themselves. And while some of those infected individuals will never develop symptoms, others will—experts estimate that it takes between two and 14 days after contracting COVID-19 for symptoms to appear. A recent update from the Centers for Disease Control and Prevention (CDC) estimates that 50 percent of coronavirus transmission occurs within that period—when infected individuals are pre-symptomatic.

In May, the CDC used modeling to plot out five possible scenarios for the trajectory of the virus, also showing the potential effectiveness of certain mitigation techniques. At that time, the agency's "best estimate" was that 40 percent of transmissions occur prior to symptom onset. Now, the scenarios have been updated with data collected through June 29, and the CDC estimates that an additional 10 percent of transmissions occur when individuals are pre-symptomatic. Currently, the agency believes that half of all transmission happens before symptom onset.

People wearing masks at work

The CDC notes that "the percentage of transmission occurring prior to symptom onset will be influenced by how quickly and effectively both symptomatic people and the contacts of known cases are quarantined." For example, someone who is pre-symptomatic for two weeks and does not social distance or take other precautions will likely account for more transmission than someone who suspects they've been exposed and immediately starts to self-quarantine. If you think that you've been exposed to the virus, it's essential that you self-quarantine for at least two weeks—this new estimated percentage makes that step even more vital.

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The CDC emphasizes that these scenarios are only projections and notes that tracking pre-symptomatic and asymptomatic cases is difficult. But though the percentages may not be completely accurate, pre-symptomatic spread is considered to a significant contributor to the lightning-fast spread of the pandemic.

For more updated guidance, check out The Number of COVID Patients Without Symptoms Is Growing, CDC Says.

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