There may be tens of times more people who have been infected with or recovered from the coronavirus in San Mateo County than the county has reported, said Dr. Scott Morrow, the county's top health officer, in an April 13 memo.

Morrow said he estimates that about 2% or 3% of the population has been infected with, or has recovered from, the coronavirus. That's roughly 15,000 to 25,000 people in San Mateo County, he said in the memo.

And roughly 1% of the population, he added, is likely capable of transmitting the virus. That translates to about 5,000 to 7,000 people countywide.

By contrast, only 721 people have tested positive for COVID-19 countywide, as of April 13. Twenty-one have died.

That means that there could be between 21 and 35 times more people who have been infected with or recovered from the coronavirus than the county is reporting on its data dashboard.

That said, the residents of San Mateo County do appear to be following the shelter-in-place order, which has been very successful at stabilizing the rate of infection locally.

But that doesn't mean things can return to normal just yet, he explained.

"If we don’t take our next steps carefully, we will experience the worst of what this virus has to offer," he wrote. "What we’re being faced with, in our immediate future, are trade-offs of the most significant kind."

It also doesn't help that an effective vaccine or effective medical treatments are unlikely to become available in the short- to medium-term.

The county's task, he said, is to increase the level of immunity to the coronavirus in the community "slowly and methodically while minimizing death, with equity in mind, while not overloading the healthcare system and minimizing economic damage."

The public health goal in the war against the novel coronavirus is to reach a "herd immunity" level, which means that roughly 70% to 80% of the population has developed immunity to the coronavirus. With only an estimated 2% or 3% of the population infected or recovered from the coronavirus and no vaccine ready, "we have a long way to go," he said. "There are no quick fixes."

But even his best estimates, he explains, are based on limited information and may be imprecise. They could be off by a factor of two or three – but probably not by a factor of 10. If that were the case, the health system would probably have already been overloaded, he said.

He said he's skeptical of some of the information the county has provided publicly because that data – confirmed cases, number of patients tested, is incomplete. The data are limited because many characteristics of the virus are unknown and testing remains very constrained in San Mateo County. The county's data panel on hospitalizations is more reliable, he added.

"The data we have, if it were to be presented to you on a more granular level, would be misleading, and I believe, downright deceptive," he said.

The latest estimates, he reports, "are likely to be more accurate than the numbers we are sharing on our website. I know that sounds ridiculous, but these estimates are better than the direct counts that I can currently provide you. That’s the status of our testing data at the moment."

Instead, he suggested, people should focus on the facts as they do what they can to stay safe and healthy.

"This virus appears to be wildly transmissible especially within households or congregate settings. Your risk from contracting the infection from any human you encounter in San Mateo County and outside your immediate household continues to be substantial unless you take all the recommended actions to protect yourself."

As for what's next, he said, "There is no playbook for the decisions we face or the balance we should attempt to maintain between these competing interests."

"One thing I do know," he added, "is that releasing the restrictions on movement and gatherings too soon, or in not an incremental enough way, will diminish the gains we’ve made and will unleash the very thing we are attempting to avoid."