Some public health experts are concerned about CDC's decision to track only breakthrough infections that result in hospitalization or death, saying the agency may as a result make decisions based on flawed or incomplete data—especially as breakthrough infections become more common amid the delta variant's spread.

How much worse will the 'delta surge' get? Watch these 7 factors.

Breakthrough infections on the rise

According to data from CDC, breakthrough infections are becoming more common as the delta variant spreads.

How much worse will the 'delta surge' get? Watch these 7 factors.

"Recent data makes clear that protection against mild and moderate disease has decreased over time," Surgeon General Vivek Murthy said. "This is likely due to both waning immunity and the strength of the widespread delta variant."

As a result, experts told USA Today vaccinated people need to reconsider their behavior in public, including going back to masking, social distancing, and avoiding large crowds.

According to Lucy Horton, an infectious disease specialist at the University of California-San Diego, the delta variant requires a "layering of protection." Covid-19 vaccines are like an umbrella, and earlier variants of the coronavirus are like a rainstorm, Horton said.

By comparison, delta is a "hurricane," Horton said.

The flaws in CDC's breakthrough infections data

In May, CDC announced it would largely no longer track breakthrough infections unless they resulted in a hospitalization or death. Some experts questioned this decision, especially as the agency sets policies around booster shots and other measures to address the latest surge.

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

In an emailed statement to ProPublica, CDC said the decision to focus on serious cases of breakthrough infections was made because officials believed a more targeted collection of data would better inform "response research, decisions, and policy."

The 6 biggest Covid-related myths we've seen, busted

CDC added that it continues to study breakthrough cases, but in a different way. "This shift will help maximize the quality of the data collected on cases of greatest clinical and public health importance," CDC said.

However, when CDC announced it would be tracking only severe cases of breakthrough infection, local and state health departments were left to their own devices to track infections. This resulted in "a patchwork of information between states," according to Jen Kates, SVP and director of global health and HIV policy at the Kaiser Family Foundation.

And much of the data that is collected by CDC is flawed, sources told Politico. Over a dozen state officials told Politico they don't have the capacity to match hospital admission data with a patient's immunization records and are instead relying on hospital administrators to report breakthrough infections. This means the data is typically aggregated and inaccurate, missing key data such as what vaccine a person received or whether they were fully vaccinated.

"I think it would be really challenging [for CDC] to interpret the results or to interpret the data when you have only some jurisdictions reporting [breakthrough infections]," Theresa Sokol, lead epidemiologist for Louisiana's state public health department, said. "I know that there are some jurisdictions that don't even have access to their vaccination data. They don't have the authority or their permission."

State officials told Politico that when they study the data sent to them, they often find errors, such as records listing the wrong gender or age. There's also a lag in data going from the states to CDC, sometimes more than two weeks. This is because electronic systems at hospitals don't automatically track breakthrough infections, which means physicians have to do so manually, Politico reports.

"We report what we have, but we know that it's limited because it's based on a direct report from a provider—as opposed to taking a data set of all hospitalizations and matching that against our vaccine registry," Sokol said. "We're not able to do that for hospitalization. We rely on individual reports from hospitals. And some report well, others do not. So we know that it's not complete."

Your top resources for Covid-19 readiness

For its part, CDC officials said they are collecting the best quality data they can under the current conditions, and that the data gaps at the state and hospital level are hampering the agency's ability to get a clear snapshot of the situation. "These issues are not going to get magically fixed overnight," one CDC official told Politico.

Breakthrough infections shouldn't deter the unvaccinated

Nonetheless, even with breakthrough infections rising—and despite the patchwork data—experts say the available evidence indicates that vaccinated people are significantly better protected from severe Covid-19 than unvaccinated people, meaning the unvaccinated should get their shots.

"When [vaccinated people] do get hospitalized, they might need a little bit of oxygen, a couple of days in the hospital, maybe a little IV fluid if they're dehydrated, and then they go home," Amy Edwards, an infectious disease specialist and associate director of infection control at University Hospitals Rainbow Babies and Children's Hospital, said. "You very rarely hear of somebody with breakthrough Covid on a ventilator."

Rupali Limaye, an associate scientist at the Johns Hopkins Bloomberg School of Public Health, said when people hear about breakthrough infections, they often assume the problem is worse than it is.

"If we take this and look at it at a population level, the risk of you having a breakthrough infection is really, really, really quite low. However, your perception of that risk changes. If you're like, 'Well, my friend got it,'" Limaye said. "You essentially overestimate what is your level of susceptibility to a breakthrough infection."

What's important is that, even if breakthrough infections happen, the vaccines are protecting people from death, Limaye said.

"Is the vaccine still doing what we really want it to do, which is prevent death? The answer is yes," Limaye said. "That's really what we should be measuring for any advocacy of any therapeutic product: Does it prevent you from dying? That's more important than could you still get it but get a mild case of it." (Rodriguez et al., USA Today, 8/19; Deam/Fortis, ProPublica, 8/20; Banco, Politico, 8/25; Mendoza, USA Today, 8/25)