In the wake of the COVID-19 pandemic and amid a tall wave of respiratory viruses, health officials in Colorado and Minnesota documented an unusual spike in deadly, invasive infections from Streptococcus bacteria late last year, according to a study published this week by the Centers for Disease Control and Prevention.

The spike is yet another oddity of post-pandemic disease transmission, but one that points to a simple prevention strategy: flu shots.

The infections are invasive group A strep, or iGAS for short, which is caused by the same group of bacteria that cause relatively minor diseases, such as strep throat and scarlet fever. But iGAS occurs when the bacteria spread in the body and cause severe infection, such as necrotizing fasciitis (flesh-eating disease), toxic shock syndrome, or sepsis. These conditions can occur quickly and be deadly.

Usually, iGAS is rare. But in the aftermath of pandemic disruptions, there have been unusual increases in multiple countries. In December, the World Health Organization noted that five countries in Europe have reported unusual increases in cases—along with a number of deaths. The list includes the UK, which has seen at least 285 deaths (29 in children aged 10 and under) in England during the 2022–2023 iGAS wave, according to the latest data. The four other European countries are France, Ireland, the Netherlands, and Sweden.

The CDC has also noted an unusually high and early rise of iGAS cases in children at the end of 2022. In the new study, local health officials report surveillance site data from the Denver metropolitan area and the state of Minnesota. Between October 1 and December 31, 2022, there were 34 iGAS cases in children from the two surveillance sites, including two deaths. That's a jump from pre-pandemic times but not from cases seen during the height of the pandemic, when incidence was unusually low. In the same three-month span in years before the pandemic (2016 to 2019), the average number of cases for the two sites totaled just 11 cases. But from 2020 to 2021, amid pandemic mitigation efforts, the average was just four cases. From 2016 to 2021, there were five iGAS deaths total.

The officials found that the bump in 2022 didn't appear linked to an unusual strain of strep A or new drug resistance. But they did highlight that it coincided with waves of respiratory viruses—namely flu and RSV (respiratory syncytial virus).

Lethal links

This isn't surprising, as group A strep is known to piggyback on viral infections. Historically, researchers have linked group A strep to the spread of chickenpox. And chickenpox vaccination has been shown to lower rates of group A strep infections in children. But researchers have noted that other viral infections can pave the way for group A strep as well.

It's unclear why some viral infections are linked to group A strep. As Ars has reported before, researchers have hypothesized that it might involve factors such as overlapping timing in disease cycling, similar transmission routes (e.g., respiratory), and the common age of victims. There's also the possibility that some viral infections use the same methods as strep to suppress human immune responses and start an infection. Thus, if a virus gets there first, it can make it easier for strep to get a foothold. Such a viral-bacterial linkage has been seen elsewhere; prior to vaccination campaigns, measles virus waves were closely linked to rises in whooping cough caused by the bacterium Bordetella pertussis. Measles is known to broadly dampen immune responses, leading to what some researchers describe as "immune amnesia."

In this season's unusual rise of iGAS, health officials suggest that the waves of flu and RSV may have enabled the bacteria's spread and disease potential. Case reporting shows a tight link between both viruses' spread and cases of iGAS in past years, as well as last year. And previous studies have also found that respiratory viruses and flu, specifically, can increase the risk of iGAS. Among the 34 iGAS cases in 2022, 15 had positive test results for one or more viral infections during or before their iGAS case, including six for flu, six for RSV, and three for SARS-CoV-2. (It's unclear how many of the 34 children were tested for viral infections. Only positive results were reported.)

"Increased activity of respiratory viruses, in combination with reduced exposure to [group A strep] and associated development of protective immunity to common [strep A] types during the COVID-19 pandemic, might have predisposed children to iGAS infection when pandemic restrictions were lifted," the authors concluded. But the findings hint at a possible prevention strategy. "Influenza vaccination might reduce the risk for iGAS, as has been demonstrated for varicella vaccination," they write.

Though a vaccine against RSV is not yet available, several are in the works.