A small portion of the millions of Americans vaccinated against the coronavirus have had "breakthrough infections," testing positive for the virus despite being fully vaccinated—a rare but not unexpected occurrence, according to public health officials.

Your top resources on the Covid-19 vaccine

Breakthrough rate is low—and not surprising

CDC, which has been tracking breakthrough cases since February, on Wednesday said roughly 5,800 people who have been vaccinated against the coronavirus have subsequently contracted the virus, with about 396—or 7%--requiring hospitalization and 74 people passing away. About 29% of the infections were asymptomatic.

Toolkit: Covid-19 vaccine communications readiness assessment

According to CDC, the data hasn't revealed any "unexpected patterns…in case demographics or vaccine characteristics." However, the agency noted that while breakthrough cases "were reported among all people of all ages eligible for vaccination," a "little over 40% of the infections were in people 60 or more years of age."

The agency said it has "developed a national Covid-19 vaccine breakthrough database where state health department investigators can currently enter, store, and manage data for cases in their jurisdiction." Further, CDC said it would continue to monitor "reported cases for clustering by patient demographics, geographic location, time since vaccination, vaccine type or lot number, and SARS-CoV-2 lineage."

However, according to the Washington Post, there are some challenges in collecting accurate information about breakthrough cases. For instance, people who've been vaccinated may be less likely to get tested if they get symptoms of Covid-19, writing off a stuffy nose or cough to a cold rather than Covid-19. And even some of the post-vaccination fatalities are still under investigation to determine if they were caused by Covid-19 or something else.

Overall, however, experts say the breakthrough rates reported by individual states and by CDC are neither unexpected nor particularly alarming. In fact, public health experts say a breakthrough rate this low is "encouraging" because it demonstrates just how effective the currently authorized vaccines have been in shielding people from infection and hospitalization.

"Vaccine breakthrough infections make up a small percentage of people who are fully vaccinated," CDC said, adding that the agency continues to advise "that all eligible people get a Covid-19 vaccine as soon as one is available to them."

"There's nothing there yet that's a red flag," Anthony Fauci, President Biden's chief medical adviser for the pandemic, said last week, in response to breakthrough case data released by some individual states. "That number of individuals who were breakthrough infections is not at all incompatible with a 90-plus percent vaccine efficacy."

Webinar series: 'Stay Up to Date' on the latest with Covid-19

Separately, Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia, said while it's important to keep an eye on those vaccinated people who develop severe illness, "[i]t's not at all surprising that in a population of hundreds of millions of people that there would still be people who get infected, who suffer mild or moderate disease."

Vaccines are (still) very effective, experts say

According to the Post, there's no clear-cut explanation as to why some people remain susceptible to the virus after vaccination. However, Fauci on Friday hypothesized that some people, such as the elderly and those with underlying health conditions, may have a weak immune response to the vaccine—an issue that has more to do with the characteristics of an individual patient than the overall efficacy of a given vaccine. "I don't think that there needs to be any concern about any shift or change in the efficacy of the vaccine."

According to the Post, while none of the currently authorized vaccines are 100% effective at preventing infection, research last year indicated that the Pfizer-BioNTech and Moderna vaccines had nearly 100% efficacy at preventing hospitalization. In addition, studies suggest that in real-world conditions, the vaccines are about 80% effective at reducing the odds of infection after the first shot, and about 90% effective after the second dose.

And although conducted at a different time, when more variants had emerged, research on Johnson & Johnson's vaccine—whose use federal officials are currently pausing—presented 100% efficacy rate against hospitalization and death, as well as a 85% efficacy rate against severe illness, Becker's Hospital Review reports.

Good? Bad? Ugly? We've updated our take on what's next for the epidemic.

Moreover, research indicates that the protection afforded by these vaccines could last for a substantial amount of time, with at least one study finding that fully vaccinated individuals had high antibody levels more than six months after being inoculated. Emerging research suggests the vaccines may also limit how infectious vaccinated people are if they do contract the virus, the Post reports.

Variants are concerning, but likely not a driving force of breakthrough cases

Another possibility, according to the Post, is that some coronavirus variants are evading the immune response induced by the vaccines. However, it's difficult to examine that theory because of significant testing data gaps that make it difficult for researchers to collect the necessary genetic sequencing data on breakthrough cases, the Post reports.

Still, based on what research has been conducted, experts think variants likely aren't the driving force behind these breakthrough infections, the Post reports, nor have officials spotted any change to the breakthrough rate since the variants became more common in the United States.

All available research suggests that the currently authorized vaccines are "quite effective" against all well-known variants of the virus, the Post reports. According to Becker's Hospital Review, early research indicates that "four of...CDC's five 'variants of concern' show minimal to moderate reduction in neutralization by antibodies generated during previous infection or vaccination."

3 big mistakes in your Covid-19 vaccine strategy (and how to fix them)

"[C]urrently, there is no evidence that Covid-19 after vaccination is occurring because of the changes in the virus," CDC spokesperson Kristen Nordlund said.

Experts urge continued vaccination

Overall, the Post reports, experts' current concerns about the breakthrough cases are not the variants or the vaccines, but how they might influence the public's willingness to get vaccinated. "What worries me more than the variants are those who choose not to vaccinate," Offit said.

Noting that there's "nothing surprising" about the breakthrough rate, Francis Collins, director of the National Institutes of Health, added, "These are still vaccines that are fantastically safe and effective. But 95% [efficacy] is not 100%."

He voiced concern that people opposed to the vaccines will use breakthrough cases to claim the vaccines don't work—an argument he pushed back on. "Would you stop wearing your seat belt because you heard somebody who was wearing a seat belt got run over by an 18-wheeler and didn't survive?" he asked.

For its part, CDC not only recommended people get the vaccine as soon as they are eligible to do so, but it continued to advise those who have been fully vaccinated to "keep taking precautions in public places, like wearing a mask, staying at least six feet apart from others, avoiding crowds and poorly ventilated spaces, and washing their hands often" (Tinker/Fox, CNN, 4/14; Sun/Achenbach, Washington Post, 4/9; Weixel, The Hill, 4/12; Bean/Masson, Becker's Hospital Review, 4/12).