Many countries in Africa do only very, very, very few tests for COVID.

[more data on this in our database: https://ourworldindata.org/explorers/coronavirus-data-explorer?zoomToSelection=true&time=latest&pickerSort=asc&pickerMetric=location&Metric=Tests&Interval=7-day+rolling+average&Relative+to+Population=true&Align+outbreaks=false&country=IND~COD~ZAF~ZWE~ZMB~KEN~ETH~SSD~UGA~NGA~MOZ~MDG~NAM~TGO~GHA~CIV~SEN~MAR~TUN~USA~DEU~FRA~CHL~ITA]

The chart above showed the latest data on daily tests.

This is the number of cumulative tests since the beginning of the pandemic.

[also on this metric you find more data in our database: https://ourworldindata.org/explorers/coronavirus-data-explorer?zoomToSelection=true&time=latest&pickerSort=asc&pickerMetric=location&Metric=Tests&Interval=Cumulative&Relative+to+Population=true&Align+outbreaks=false&country=IND~COD~ZAF~ZWE~ZMB~KEN~ETH~SSD~UGA~NGA~MOZ~MDG~NAM~TGO~GHA~CIV~SEN~MAR~TUN~USA~DEU~FRA~CHL~ITA~DNK]

While I generally think that changes in the positive-rate are a good way to monitor whether a country tests adequately I'm not convinced this is the case for all countries, especially for poor countries.

It could also be that only very few people have the chance to get tested.