The U.S. Centers for Disease Control and Prevention on Tuesday warned that coronavirus will penetrate U.S. borders and spread within communities, despite containment efforts, so families should prepare for disruptions such as schools and workplaces closing.

“It’s not so much of a question of if this will happen in this country anymore but a question of when this will happen,” physician Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said during a teleconference.

At the same time, she acknowledged that testing capacity remains very limited because a diagnostic test kit developed by the CDC and sent to state labs 19 days ago had a faulty reagent, a substance used in a chemical reaction. Health departments in New Jersey and Pennsylvania said Tuesday that they had yet to receive working kits.

Only a dozen U.S. labs outside the CDC have usable tests. The CDC’s website says it has tested only 426 people who meet narrow criteria — respiratory symptoms and recent travel to China or close contact with a confirmed case. Some experts worry that undetected infection may be spreading because many people who get sick have only mild symptoms.

Messonnier said a new version of the test will be shipped “soon.”

“I’m frustrated,” she said, but added that “we have to make sure we keep to the highest level of quality assurance.”

The new virus, which causes an upper respiratory illness that ranges from mild to deadly, has spread to 33 countries since it emerged in Wuhan, China, in late December, and more countries report infections almost daily. Although most of the 80,239 confirmed cases are in China, the World Health Organization on Monday said the epidemic could become a pandemic, defined as an outbreak of a new disease-causing germ that spreads easily, person-to-person, around the globe. So far, 2,700 people have died, including 34 outside of China, according to WHO.

The CDC has issued guidelines to try to minimize community spread, such as canceling big gatherings and closing schools, but Messonnier warned that disruptions to daily life “may be severe.” She said families should think about issues such as child care, missing work, and telecommuting.

“I understand this whole situation may seem overwhelming,” she said. “But these are things that people need to start thinking about now. I had a conversation with my family over breakfast this morning, and I told my children that, while I don’t think they are at risk right now, we as a family need to be preparing for significant disruption of our lives.”

The CDC’s test was authorized for emergency distribution but has not gone through the normal government approval process. The test uses PCR (polymerase chain reaction), a Nobel Prize-winning technology that has revolutionized molecular biology since its invention in 1986. It creates unlimited copies of DNA using an original strand from a sample of blood or other body fluid.

Big companies such as Roche have automated components of PCR tests to make them faster, less laborious, less prone to cross-contamination, and usable in hospitals and clinics. The CDC’s test, in contrast, has room for error.

“The CDC came up with a stopgap,” said Michael Mina, an epidemiology and infectious-disease specialist at Harvard University. “But they sent a conventional PCR kit that would be used in research labs, not a plug-and-play cartridge. There are some tubes that require filling. And the CDC isn’t in the business of making reagents.”

Countries with sizable outbreaks, including South Korea, have come up with coronavirus tests and run them on thousands of samples.

In the United States, dozens of companies and academic labs are racing to develop better tests. Among the innovators are Biomeme, a Philadelphia biotech start-up, and Haim Bau, a computational and fluid mechanics expert at the University of Pennsylvania.

But obtaining samples of the virus to validate new tests is a major obstacle for many developers.

“If we can, we will try to test our assay in China with samples from there,” Bau said.

Max Perelman, cofounder of Biomeme, said the company has shared its PCR test technology — a handheld platform that runs on a smartphone — with the U.S. Food and Drug Administration, although the company has not given its assays for COVID-19 testing.

“They’re very tight-lipped," Perelman said, about plans for emergency use authorization of COVID-19 tests other than the CDC’s.

Messonnier on Tuesday said commercial tests are “coming,” but not how soon.

Meanwhile, the Association of Public Health Laboratories, which represents state and local laboratories, has sent a letter asking the FDA for permission to develop and use their own tests, according to the Washington Post.

“This is an extraordinary request, but this is an extraordinary time,” Scott Becker, chief executive of the association, told the Post.

In an earlier briefing, the World Health Organization announced findings from its experts who have been deployed in China:

  • Aggressive quarantines and other restrictions in that country likely prevented hundreds of thousands of cases.
  • The genetic code of the virus has not mutated much since it was initially identified, suggesting that ongoing efforts to produce a vaccine will be useful.
  • Among people in Wuhan who were known to be infected with the virus, the fatality rate was between 2% and 4%, and 0.7% outside Wuhan. But epidemiologists say the true fatality rates are likely much lower, as many people who were infected were never identified.
  • Children have not been hard-hit by the illness. Most fatalities have occurred in those aged 70 and up. In addition to age, underlying illness and smoking also increase the risk of severe symptoms.
  • People with mild symptoms have recovered after two weeks. Those with a severe or critical form of the illness, if they recover, do so within three to six weeks.

Staff writer Tom Avril contributed to this article.