Chicago Public Schools, the nation's third largest school district, canceled instruction on Friday for the third day in a row amid an impasse over in-person learning.

Classes were first canceled Wednesday, and then again on Thursday, after a majority of the Chicago Teachers Union's membership voted this week in favor of remote learning during a surge in COVID-19 cases.

The district said Thursday evening that all classes will be canceled on Friday, though some schools may be able to offer in-person activities for students.

"Our schools are the best, safest place for students to be during this pandemic, and we are working tirelessly to get everyone back in class every day," Pedro Martinez, head of the school district, said in a letter to families, noting that they are continuing to work with the union "to resolve this situation."

The teacher's union is calling for more robust school COVID-19 testing and contact tracing.

Johnson & Johnson's single-shot COVID-19 vaccine offers lower initial efficacy compared to Pfizer's and Moderna's, but protection against breakthrough infection remains more stable over time, according to a new study sponsored by the pharmaceutical company.

The study found that the J&J vaccine was 74% effective against breakthrough infection in the weeks following the shot. This level of protection held steady over the next three months and started waning after the fourth month.

The Pfizer and Moderna vaccines, meanwhile, were 88% and 92% effective, respectively, against breakthrough infection in the weeks following the second dose, the study found. This level of protection started waning after the second month, falling progressively over six months.

Protection against severe illness remained more stable over time for all three vaccines.

The analysis, which has not been peer-reviewed, looked at data from 168 million people between Jan. 1 and Sept. 7 of last year. It covers a period of alpha and delta variant dominance, but the results could change now that the omicron variant is also present.

The data supports current guidance from the Centers for Disease Control and Prevention for booster shots after two months for the J&J vaccine to bring initial efficacy higher, and booster shots for Pfizer and Moderna vaccines after five and six months, respectively, to boost efficacy after a period of waning.

-ABC News' Sony Salzman

COVID-19 vaccines dramatically reduce the risk of severe illness and death from the virus, according to a new study from the Centers for Disease Control and Prevention published Thursday.

The study looked at 1.2 million fully vaccinated adults who received either two shots of the Pfizer-BioNTech or Moderna vaccines or one shot of the Johnson & Johnson vaccine.

Just 0.18% of patients had breakthrough COVID-19 infections, and 0.015% developed serious illness that led to hospitalization, ICU admission, mechanical ventilation, intubation or death.

The small portion of people who did become seriously sick or die of COVID-19 after being vaccinated were primarily older adults, immunocompromised people or those living with multiple underlying medical conditions.

The study was conducted before the emergence of the omicron variant, which appears to more easily evade --- at least partially -- the protection offered by vaccines.

-ABC News' Sony Salzman

An estimated 126,000 Americans are currently hospitalized with COVID-19, according to new data from the U.S. Department of Health and Human Services.

Of those patients, about one-sixth -- or 21,000 -- are in intensive care units.

The spread of the omicron variant is pushing the U.S. closer to the hospitalization record set last winter when 140,000 patients with the virus were hospitalized .

Meanwhile, the country is averaging 554,000 new COVID-19 cases every day, a five-fold increase from a month ago, federal data shows.

Over the last week, the U.S. has reported about 3.88 million COVID-19 infections, which averages out to about six Americans testing positive every second.

-ABC News' Arielle Mitropoulos