New Zealand's first instance of COVID-19 transmission in six months has been identified as the highly contagious delta variant.

"We are dealing with a delta variant," New Zealand Prime Minister Jacinda Ardern announced during a press conference in Wellington on Wednesday. "Our case has originated in Australia."

The case, which was detected in the community on Tuesday, prompted New Zealand to immediately impose a nationwide lockdown. More community cases have emerged since then, with the cluster growing to 10 by Wednesday afternoon, according to data from New Zealand's Ministry of Health.

Ardern said genomic sequencing has linked the initial case to an outbreak of the delta variant in neighboring Australia's New South Wales state.

"Now, the job we have is to work through how and when it got here," she said.

It's the first time that the island nation of 5 million people has confirmed the presence of the delta variant, which was initially identified in India last October. At least 148 countries around the globe have reported cases of the delta variant, according to the World Health Organization.

It's also the first time in more than a year that New Zealand has had a snap level four lockdown, the highest level of restrictions.

In total, the country has reported 2,936 confirmed cases of COVID-19 since the start of the pandemic, including 26 deaths, according to health ministry data.

Everyone in Chicago who is 2 years of age and older must wear a face mask indoors starting Friday, regardless of whether they have been vaccinated against COVID-19.

Chicago health officials announced the reinstatement of the indoor mask mandate on Tuesday, after the Windy City saw its daily average of newly diagnosed COVID-19 cases surpass 400 -- a metric that moves the city from "substantial risk" to "higher risk."

"With the highly transmissible delta variant causing case rates to increase, now is the time to re-institute this measure to prevent further spread and save lives," Dr. Allison Arwady, commissioner of the Chicago Department of Public Health, said in a statement. "We continue to track the data closely and are hopeful this will only be temporary and we can bend the COVID curve, as we’ve done in the past."

During a press conference Tuesday, Arwady noted that other COVID-19 metrics, such as the city's test positivity average and hospitalizations, remain at "lower risk."

"A high case count does not automatically translate to a high hospitalization count and a high death count," she told reporters, "and we're hopeful that having the mask in place for everybody will get us through delta while we keep working on getting folks vaccinated."

Masks will be required citywide in all indoor public settings, including bars, restaurants, gyms, hair salons, private clubs and common areas in residential buildings. As with previous mask mandates, the face coverings can be temporarily taken off for certain activities that require their removal, such as eating and drinking or for facials and beard shaves.

Masks can also be removed by employees in settings that are not open to the public, such as office cubicles, so long as the individuals are static and maintaining at least 6 feet from others. The face coverings remain mandatory on public transportation as well as in educational, health care, correctional and congregate settings.

The new mandate does not include capacity limits at public places, and masks will remain optional in outdoor settings.

"We are not anticipating, at this point, adding additional business restrictions. However, we're watching what happens with these metrics," Arwady told reporters. "Our goal is to remain open but careful."

An Alabama doctor is seeing young, healthy patients die from COVID-19 amid the surge of the delta variant.

Dr. Jenna Carpenter, a pulmonary care physician at Marshall Medical South in Guntersville, Alabama, has lost three patients under the age of 40 in the past week from complications related to COVID-19, she told ABC Huntsville, Alabama, affiliate WAAY.

"The young man I lost this week was perfectly healthy," she said. "He wasn't overweight. He did not have any known medical issues and that was a tragedy."

The worst phone call the physicians have to make is to inform family members that their loved one has taken a turn for the worst, Carpenter added.

"In our heart we know this is going to be the last time these folks talk to their families," she said.

The state currently has more ICU patients than beds, and frontline workers are also getting sick from the highly contagious variant, WAAY reported.

"Last week we were down to 35 or 40 ICU beds. Now we are down at the single digits," Dr. Don Williamson, a former state health officer who is now the president and CEO of the Alabama Hospital Association, told the station. "It doesn’t matter if it is six or two, we could even be negative ICU beds."

-ABC News’ Cherise Rudy

Detroit has opened a mass vaccine site for boosters at its convention center.

The TCF Center has played an integral role for Detroit residents during the pandemic, first acting as a mass testing site, a field hospital and eventually a mass vaccination site.

It is currently the only location in the city to get a third booster shot.

-ABC News’ Cherise Rudy

A new CDC analysis found that Pfizer and Moderna vaccine efficacy dropped "significantly" among nursing home residents from March to July, as the delta variant became the predominant strain in the United States.

Researchers analyzed weekly reports from thousands of nursing home facilities in the U.S. and found that mRNA vaccines were roughly 75% effective against preventing any infection in late winter/early spring of 2021, early in the mass vaccination rollout and prior to the emergence of the delta variant. By summer of this year, effectiveness against any infection had dropped to 53%.

Crucially, this doesn't mean vaccines aren't working for nursing home residents, but the significant drop in effectiveness from March to July may support the use of booster doses for them, according to the CDC report.

A second analysis found that the mRNA vaccines are holding up well against hospitalizations for COVID-19. The research, which was conducted across 21 U.S. hospitals, found that the Pfizer and Moderna vaccines remained between 84% and 86% effective against potential hospitalizations from March to July of this year. A third analysis, conducted in New York State, found that all three authorized vaccines remained more than 90% effective at preventing hospitalization from early May to late June.

-ABC News' Sony Salzman, Eric Strauss