A new model suggests that COVID-19 was spreading in the US weeks before the first detected cases and deaths.

The first death actually happened at the beginning of February in California, according to researchers – three weeks before the first confirmed death in Washington state.

There were only 23 confirmed cases in major US cities on March 1. However, the model suggests that number was likely closer to 28,000. New York City likely had 10,000 cases by March 1 – two weeks before any social distancing guidelines.

“Those cases were not detected because if you don’t test, you don’t detect,” Dr. Alessandro Vespignani, a Northeastern University professor told CBS News. The model suggests coronavirus began its US spread in January and was increasing at a rate of 50 cases per day by mid-February.