On the heels of last month’s news of stunning results from Pfizer’s and Moderna’s experimental Covid-19 vaccines, Senator Rand Paul tweeted a provocative comparison.

The new vaccines were 90 percent and 94.5 percent effective, Mr. Paul, Republican of Kentucky, said. But “naturally acquired” Covid-19 was even better, at 99.9982 percent effective, he claimed.

Mr. Paul is one of many people who, weary of lockdowns and economic losses, have extolled the benefits of contracting the coronavirus. The senator was diagnosed with the disease this year and has argued that surviving a bout of Covid-19 confers greater protection than getting vaccinated.

Lately, in the ongoing conversation about how to defeat the coronavirus, experts have made reference to the “Swiss cheese model” of pandemic defense.

The metaphor is easy enough to grasp: Multiple layers of protection, imagined as cheese slices, block the spread of the new coronavirus, SARS-CoV-2, the virus that causes Covid-19. No one layer is perfect; each has holes, and when the holes align, the risk of infection increases. But several layers combined — social distancing, plus masks, plus hand-washing, plus testing and tracing, plus ventilation, plus government messaging — significantly reduce the overall risk. Vaccination will add one more protective layer.

With coronavirus infections soaring across the nation, federal health officials on Friday urged Americans in the most forceful language yet to take steps to protect themselves — starting with consistent, proper use of masks — and pressed local governments to adopt 10 public health measures deemed necessary to contain the pandemic.

The guidance reflected deep concern at the agency that the pandemic is spiraling further out of control and that many hospitals are reaching a breaking point, potentially disrupting health care across the country.

Agency officials have issued increasingly stark warnings in the waning weeks of the Trump administration, and President-elect Joseph R. Biden Jr. has promised a new national strategy to turn back the virus. On Thursday, Mr. Biden said he would call on Americans to wear facial coverings for 100 days.