This story first appeared at ProPublica. ProPublica is a Pulitzer Prize-winning investigative newsroom. Sign up for The Big Story newsletter to receive stories like this one in your inbox.

Meggan Ingram was fully vaccinated when she tested positive for COVID-19 early this month. The 37-year-old’s fever had spiked to 103 and her breath was coming in ragged bursts when an ambulance rushed her to an emergency room in Pasco, Washington, on Aug. 10. For three hours she was given oxygen and intravenous steroids, but she was ultimately sent home without being admitted.

Seven people in her house have now tested positive. Five were fully vaccinated and two of the children are too young to get a vaccine.

As the pandemic enters a critical new phase, public health authorities continue to lack data on crucial questions, just as they did when COVID-19 first tore through the United States in the spring of 2020. Today there remains no full understanding on how the aggressively contagious delta variant spreads among the nearly 200 million partially or fully vaccinated Americans like Ingram, or on how many are getting sick.

The nation is flying blind yet again, critics say, because on May 1 of this year — as the new variant found a foothold in the U.S. — the Centers for Disease Control and Prevention mostly stopped tracking COVID-19 in vaccinated people, also known as breakthrough cases, unless the illness was severe enough to cause hospitalization or death.

Individual states now set their own criteria for collecting data on breakthrough cases, resulting in a muddled grasp of COVID-19’s impact, leaving experts in the dark as to the true number of infections among the vaccinated, whether or not vaccinated people can develop long-haul illness, and the risks to unvaccinated children as they return to school.

“It’s like saying we don’t count,” said Ingram after learning of the CDC’s policy change. COVID-19 roared through her household, yet it is unlikely any of those cases will show up in federal data because no one died or was admitted to a hospital.

The CDC told ProPublica in an email that it continues to study breakthrough cases, just in a different way. “This shift will help maximize the quality of the data collected on cases of greatest clinical and public health importance,” the email said.

In addition to the hospitalization and death information, the CDC is working with Emerging Infections Program sites in 10 states to study breakthrough cases, including some mild and asymptomatic ones, the agency’s email said.

Under pressure from some health experts, the CDC announced Wednesday that it will create a new outbreak analysis and forecast center, tapping experts in the private sector and public health to guide it to better predict how diseases spread and to act quickly during an outbreak.

Tracking only some data and not releasing it sooner or more fully, critics say, leaves a gaping hole in the nation’s understanding of the disease at a time when it most needs information.

“They are missing a large portion of the infected,” said Dr. Randall Olsen, medical director of molecular diagnostics at Houston Methodist Hospital in Texas. “If you’re limiting yourself to a small subpopulation with only hospitalizations and deaths, you risk a biased viewpoint.”

On Wednesday, the CDC released a trio of reports that found that while the vaccine remained effective at keeping vaccinated people out of the hospital, the overall protection appears to be waning over time, especially against the delta variant.

Among nursing home residents, one of the studies showed vaccine effectiveness dropped from 74.7% in the spring to just 53.1% by midsummer. Similarly, another report found that the overall effectiveness among vaccinated New York adults dropped from 91.7% to just under 80% between May and July.

The new findings prompted the Biden administration to announce on Wednesday that people who got a Moderna or Pfizer vaccine will be offered a booster shot eight months after their second dose. The program is scheduled to begin the week of Sept. 20 but needs approval from the Food and Drug Administration and a CDC advisory committee.

This latest development is seen by some as another example of shifting public health messaging and backpedaling that has accompanied every phase of the pandemic for 19 months through two administrations. A little more than a month ago, the CDC and the FDA released a joint statement saying that those who have been fully vaccinated “do not need a booster shot at this time.”

The vaccine rollout late last year came with cautious optimism. No vaccine is 100% percent effective against transmission, health officials warned, but the three authorized vaccines proved exceedingly effective against the original COVID-19 strain. The CDC reported a breakthrough infection rate of 0.01% for the months between January and the end of April, although it acknowledged it could be an undercount.

As summer neared, the White House signaled it was time for the vaccinated to celebrate and resume their pre-pandemic lives.

Trouble, though, was looming. Outbreaks of a new, highly contagious variant swept India in the spring and soon began to appear in other nations. It was only a matter of time before it struck here, too.

“The world changed,” said Dr. Eric Topol, director of the Scripps Research Translational Institute, “when delta invaded.”

The current crush of U.S. cases — well over 100,000 per day — has hit the unvaccinated by far the hardest, leaving them at greater risk of serious illness or death. The delta variant is considered at least two or three times more infectious than the original strain of the coronavirus. For months much of the focus by health officials and the White House has been on convincing the resistant to get vaccinated, an effort that has so far produced mixed results.

Yet as spring turned to summer, scattered reports surfaced of clusters of vaccinated people testing positive for the coronavirus. In May, eight vaccinated members of the New York Yankees tested positive. In June, 11 employees of a Las Vegas hospital became infected, eight of whom were fully vaccinated. And then 469 people who visited the Provincetown, Massachusetts, area between July 3 and July 17 became infected even though 74% of them were fully vaccinated, according to the CDC’s Morbidity and Mortality Weekly Report.

While the vast majority of those cases were relatively mild, the Massachusetts outbreak contributed to the CDC reversing itself on July 27 and recommending that even vaccinated people wear masks indoors — 11 weeks after it had told them they could jettison the protection.

And as the new CDC data showed, vaccines continue to effectively shield vaccinated people against the worst outcomes. But those who get the virus are, in fact, often miserably sick and may chafe at the notion that their cases are not being fully counted.

“The vaccinated are not as protected as they think,” said Topol, “They are still in jeopardy.”

The CDC tracked all breakthrough cases until the end of April, then abruptly stopped without making a formal announcement. A reference to the policy switch appeared on the agency’s website in May about halfway down the homepage.

“I was shocked,” said Dr. Leana Wen, a physician and visiting professor of health policy and management at George Washington University. “I have yet to hear a coherent explanation of why they stopped tracking this information.”

The CDC said in an emailed statement to ProPublica that it decided to focus on the most serious cases because officials believed more targeted data collection would better inform “response research, decisions, and policy.”

Sen. Edward Markey, D-Mass., became alarmed after the Provincetown outbreak and wrote to CDC director Dr. Rochelle Walensky on July 22, questioning the decision to limit investigation of breakthrough cases. He asked what type of data was being compiled and how it would be shared publicly.

“The American public must be informed of the continued risk posed by COVID-19 and variants, and public health and medical officials, as well as health care providers, must have robust data and information to guide their decisions on public health measures,” the letter said.

Markey asked the agency to respond by Aug. 12. So far the senator has received no reply, and the CDC did not answer ProPublica’s question about it.

When the CDC halted its tracking of all but the most severe cases, local and state health departments were left to make up their own rules.

There is now little consistency from state to state or even county to county on what information is gathered about breakthrough cases, how often it is publicly shared, or if it is shared at all.

“We’ve had a patchwork of information between states since the beginning of the pandemic,” said Jen Kates, senior vice president and director of global health and HIV policy at Kaiser Family Foundation.

She is co-author of a July 30 study that found breakthrough cases across the U.S. remained rare, especially those leading to hospitalization or death. However, the study acknowledged that information was limited because state reporting was spotty. Only half the states provide some data on COVID-19 illnesses in vaccinated people.

“There is no single, public repository for data by state or data on breakthrough infections, since the CDC stopped monitoring them,” the report said.

In Texas, where COVID-19 cases are skyrocketing, a state Health and Human Services Commission spokesperson told ProPublica in an email the state agency was “collecting COVID-19 vaccine breakthrough cases of heightened public health interest that result in hospitalization or fatality only.”

Other breakthrough case information is not tracked by the state, so it is unclear how often breakthroughs occur or how widely cases are spreading among the vaccinated. And while Texas reports breakthrough deaths and hospitalizations to the CDC, the information is not included on the state’s public dashboard.

“We will be making some additions to what we are posting, and these data could be included in the future,” the spokesperson said.

South Carolina, on the other hand, makes public its breakthrough numbers on hospitalizations and deaths. Milder breakthrough cases may be included in the state’s overall COVID-19 numbers but they are not labeled as such, said Jane Kelly, an epidemiologist at the South Carolina Department of Health and Environmental Control.

“We agree with the CDC,” she said, “there’s no need to spend public health resources investigating every asymptomatic or mild infection.”

In Utah, state health officials take a different view. “From the beginning of the pandemic we have been committed to being transparent with our data reporting and … the decision to include breakthrough case data on our website is consistent with that approach,” said Tom Hudachko, director of communications for the Utah Department of Health.

Some county-level officials said they track as many breakthrough cases as possible even if their state and the CDC does not.

For instance, in Clark County, Nevada, home of Las Vegas, the public health website reported that as of last week there were 225 hospitalized breakthrough cases but 4,377 vaccinated people overall who have tested positive for the coronavirus.

That means that less than 5% of reported breakthrough cases resulted in hospitalization. “The Southern Nevada Health District tracks the total number of fully vaccinated individuals who test positive for COVID-19 and it is a method to provide a fuller picture of what is occurring in our community,” said Stephanie Bethel, a spokesperson for the health district in an email.

Sara Schmidt, a 44-year-old elementary school teacher in Alton, Illinois, is another person who has likely fallen through the data hole.

“I thought, ‘COVID is over and I’m going to Disney World,’” she said. She planned a five-day trip for the end of July with her parents. Not only had she been fully vaccinated, receiving her second shot in March, she is also sure she had COVID-19 in the summer of 2020. Back then she had all the symptoms but had a hard time getting tested. When she finally did, the result came back negative, but her doctor told her to assume it was inaccurate.

“My guard was down,” she said. She was less vigilant about wearing a mask in the Florida summer heat, assuming she was protected by the vaccination and her presumed earlier infection.

On the July 29 plane trip home, she felt mildly sick. Within days she was “absolutely miserable.” Her coughing continued to worsen, and each time she coughed her head pounded. On Aug. 1 she tested positive. Her parents were negative.

Now, three weeks later, she is far from fully recovered and classes are about to begin at her school. There’s a school mask mandate, but her students are too young to be vaccinated. “I’m worried I will give it to them, or I will get it for a third time,” she said.

But it is doubtful her case will be tracked because she was never hospitalized. That infuriates her, she said, because it downplays what is happening.

“Everyone has a right to know how many breakthrough cases there are,” she said, “I was under the impression that if I did get a breakthrough case, it would just be sniffles. They make it sound like everything is under control and it’s not.”