Covid-19 had already infected 1.3 million Kenyans by June - a far higher number than previously thought.
That is the finding of a landmark study by the Kenya Medical and Research Institute (Kemri).
Scientists analysed national blood donations given between May and June - and found that more than 4% of people had antibodies, which is a sign of having Covid-19 and recovering.
More than 3,000 people gave blood over a period of six weeks at the start of the pandemic. None of them were showing any symptoms of Covid-19.
Samples of their blood were analysed by scientists at Kemri.
They discovered more than one in 25 had already been infected with the virus - and recovered, most of them without showing any symptoms.
In the capital Nairobi and the coastal city Mombasa - the early hotspots - that figure was one in 12.
Researchers say this means that Kenya most likely reached its first peak far earlier than initially thought.
This is the first accurate picture of the pandemic in Kenya. Similar studies have been done in Mozambique and Malawi.
More than 65,000 people in Kenya have contracted Covid-19, with over 1,000 deaths.