This mean that getting the pandemic under control is that much harder, even if cases are generally not severe.
Despite a rather unpleasant bout with COVID-19 this past January, Jennifer McDonnell was relieved to think that she, and her family, could finally return to normal life, with a potentially strong immune protection stemming from both vaccination and a past infection.
"We had been cautious, but once we [got COVID-19] we thought, 'We're good,'" McDonnell, who lives in Illinois, told ABC News. "We were vaccinated, boosted. We had natural immunity. We thought that we didn't need to wear a mask."
However, four months later, McDonnell was met with an unwelcome surprise, when she awoke in the middle of the night to a high-grade fever and a sore throat.
"I felt so terrible. I was so sick," McDonnell said.
Because of her previous coronavirus infection just a few months earlier - the Centers for Disease Control and Prevention (CDC) recommends testing for those who develop symptoms within 90 days of recovering - McDonnell thought she must have caught the flu, or perhaps come down with strep throat. But when her tests came back negative, her doctor decided to test her for coronavirus.
"When he told me that I was positive for COVID-19 again, I thought, 'You've got to be kidding me,'" McDonnell said. "I thought you know six months, I'm good. I don't need to wear a mask. We were at like a whole bunch of sporting events... at parties without masks, no social distancing - nothing - we thought we had time."
Now, three weeks after testing positive, McDonnell said that she still does not feel like herself, with this second round of COVID-19 feeling "way worse" than the first.
Although she was shocked to find herself positive again, data reveals that McDonnell is not alone. The phenomenon of reinfection -- which the CDC defines as a person who was infected, recovered, and then later became infected again -- has become increasingly reported over the last year, at first during the delta variant surge, and then during subsequent omicron surges.
Reinfection, spurred in part by more transmissible variants, waning immunity and relaxed attitudes towards effective mitigation measures like masks, mean that getting the pandemic under control is that much harder, even if cases are generally not severe, experts said. It also indicates that notions of "protection," both from natural infection and vaccination, held up earlier in the pandemic have changed. Similar to a common cold, it is possible to become infected multiple times.
ABC News contacted officials from health departments in all 50 states, and found that although data is still largely unavailable, a growing number of people do appear to experience reinfection with COVID-19.
More than 1.6 million cases of reinfection have been reported across 24 states since data collection began, according to an ABC analysis and compilation of state-provided data.
New York, Maine and California, have all reported over 200,000 reinfections respectively, while Michigan and North Carolina have confirmed over 100,000 reinfection cases each.
States reported that an increasing proportion of people had been reinfected two times, with some people facing reinfection three, four, and on very rare occasions, even five times. Definitions of reinfection vary by state.
Although health officials stressed that reinfections are likely significantly undercounted due to lack of reporting and home testing, overall, reinfections still represent a small share of the total number of cases reported across states.
The percent of cases confirmed as reinfections varies widely by jurisdiction, but a handful of states that provided the data reported a range of between approximately 4% and 15% of cases, the highest reported being in Utah in recent weeks.
"I believe that virtually everyone will be infected at some point in their lives, and most people multiple times. We see plenty of patients with reinfection, though it is a really hard number to quantify," Dr. Shira Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center, told ABC News.
In May, the Centers for Disease Control and Prevention (CDC) acknowledged that despite some protection against infection provided through prior infection and vaccination, cases of reinfection are possible and even can occur within 90 days of an initial infection.
"Studies suggest that reinfection with SARS-CoV-2 with the same virus variant as the initial infection or reinfection with a different variant are both possible," the CDC wrote.
Symptoms during reinfection have been found to be less severe than during the initial infection, the CDC wrote, but some individuals may still experience more severe forms of disease.
It is also unclear how many people, who became infected during the omicron surge, have become reinfected by the same variant or subvariant of omicron.
"Although rates of reinfection might change as new variants emerge, vaccination remains the safest strategy for preventing future [COVID-19] infections, hospitalizations, [long-COVID-19] and death," officials said. Isolation and treatment are still recommended for people who experience reinfection.
Doron explained that it can be difficult for physicians to tell the difference between a true reinfection and a "re-positive," which she described as people staying positive on PCR tests for months at a time, or the occurrence of new symptoms while residual viral RNA is leftover in the nose, which can cause people to think they have tested positive again.
"The frequency of false positive test results is under-appreciated, so I often repeat the test in patients who are presenting with what appears to be a reinfection within a relatively short period of time - like 6 months - and frequently that repeat test is negative, but when it's positive, we don't have a way to know if the earlier one could have been a false positive," Doron explained. "Reinfection does happen."
Reinfection is more likely to happen to an individual when a different and more immune evasive variant is circulating, health experts said. The most recent examples were first the delta variant, and in the months that followed, the omicron variant.
"Although the increased ability to evade existing COVID-19 immunity began with the delta variant, it was omicron that took things to a whole new level," Dr. Maureen Miller, professor of epidemiology at Columbia University's Mailman School of Public Health, told ABC News. "The problem is compounded by the waning protection afforded by vaccination, previous infection or a combination of the two. Sadly antibodies wane in a period of three to six months."
In New Jersey, unvaccinated people were 1.7 times more likely to become reinfected than vaccinated people, and 7.3 times more likely to become reinfected than vaccinated and boosted people.
With BA.2 subvariant, BA.2.12.1, on the rise, and other omicron sub variants, BA.4 and BA.5, looming behind, Miller said that there are growing concerns that emerging variants will be able to continue to evade immunity to an even greater degree.
"Each new variant and subvariant and sub-subvariant is going to have different ability to break through prior immunity, and prior immunity from vaccination and infection is so variable between individuals," Doron explained.
Although reinfection likely will occur more frequently, Miller added that thus far, the current surge has not been accompanied by the high levels of hospitalizations and deaths though "both will be unacceptably high as we move through this latest wave."
Although the latest COVID-19 variants have been shown to be more transmissible and better at evading vaccines, thus far, they have not been found to be more severe.
McDonnell said that she feels that many people, like her, have a false sense of protection after the first infection.
Further, she said, "nobody [where she lives] wears a mask. If you have a mask on, people look at you like, why are you wearing a mask? Because they dropped all the mandates," adding, "everyone just assumes it's over. It's not."
How long people are truly protected against reinfection, following a COVID-19 diagnosis, is still a major question on the mind of many scientists, Doron said.
"If I just had COVID, for how long can I drop the precautions before I need to start to be careful again because I am at risk of reinfection? The answer is not very clear," Doron explained.
In addition, the definition of "protection" against COVID-19, through natural immunity or vaccination, has changed frequently throughout the pandemic. In the initial months after the vaccine rollout, federal officials widely touted that vaccination largely protected against all forms of infection - mild, severe, and death - as well as re-infection.
"The science demonstrates that if you are fully vaccinated, you are protected," CDC Director Dr. Rochelle Walensky said during a press briefing in May 2021, following the agency's decision to drop its mask recommendation for fully vaccinated people.
Breakthrough infections - vaccinated people getting COVID-19 - were exceedingly rare, let alone reinfections.
Soon after, the delta variant emerged and new data was released, regarding the risk of breakthrough infections and the possibility of transmission between fully vaccinated people. Although protection against severe disease and death appeared to remain relatively steady, it became evident that people could not be unequivocally protected against infection following vaccination or infection.
In the early days of the pandemic, reaching herd immunity was frequently discussed by public health experts as a critical long-term goal in achieving national protection against COVID-19 and returning to normalcy.
However, this past March, Dr. Anthony Fauci, who had also often pointed to the importance of herd immunity, published a perspective paper in the Journal of Infectious Diseases in which he said that achieving "classical herd immunity" may not be achievable.
"There are significant obstacles to achieving complete herd immunity with COVID-19," Fauci and his co-authors wrote. "Neither infection nor vaccination appears to induce prolonged protection against SARS-CoV-2 in many or most people."
The seemingly never-ending cycle of infection and reinfection can be perceived as daunting as the nation pushes to move on to a phase of the pandemic in which people are truly able to return to their normal lives.
"Remember, 'flatten the curve'? We have not reached a threshold where enough people have a level of immunity that we can consider COVID-19 to be under control. As long as there is not enough immunity, it gives the virus a chance to mutate even more," Miller said.
Although the U.S. is in a "much better" position than it was at the onset of the pandemic, in terms of immunity, Miller stressed that Americans must still be responsive to the rising and falling COVID-19 waves that will "continue to infect and reinfect us until we can achieve a level of control, where our health care systems are not periodically overwhelmed and deaths from COVID-19 become a much less common occurrence."
Doron noted that the best ways for people to protect themselves against reinfection remain the same: stay up to date on vaccination, wear a high-quality mask in crowded indoor spaces, particularly if you are at increased risk.
"We will never eradicate or eliminate COVID-19. We'll have to settle for controlling it. We're not there yet," Miller added.