The most populous US state has a total of 400,166 COVID-19 cases, putting it on the verge of surpassing New York - the original epicenter of the nation's outbreak - for the highest number of infections since the novel coronavirus was first detected in the United States in January.

If California were a country, and it overtakes New York, it would rank fifth in the world for total COVID-19 cases behind only the United States, Brazil, India and Russia. New York currently has over 412,800 total cases and is adding on average 700 new cases a day in July. In California there is an average of 8,300 new cases a day.

The rapid increase of cases has made it difficult to trace the pathogen's path through the community through contact tracing, a process of interviewing people who test positive for the virus to find out how they were exposed, and whom they in turn might have exposed, said California Secretary of Health and Human Services Mark Ghaly.

"No one anticipated building a program to contact trace the number of cases we’re seeing here," Dr Ghaly said at a news conference, referring to Los Angeles and other counties struggling to trace cases of the disease.

The nation's top infectious disease expert, Dr Anthony Fauci, said on CNN Tuesday the main issue with testing is that results often do not come back for days, making it too late to do a proper job of contact tracing.

Since its crush of cases earlier this year, New York state has gotten the virus under control, reporting the fewest hospitalizations in four months on Monday.

More than 141,000 Americans have died of COVID-19 - nearly a quarter of the global total - and the US Centers for Disease Control and Prevention has forecast that the death toll could reach 157,000 by August 8.