These other viruses – including influenza A, influenza B, parainfluenza, norovirus, respiratory syncytial virus (RSV), human metapneumovirus – all appear to circulate at or near levels lower than ever measured. The same goes for the respiratory bacteria that cause whooping cough, better known as whooping cough and pneumonia.
“It’s crazy,” said Lynnette Brammer, who heads the Centers for Disease Control and Prevention’s domestic flu surveillance team. “This is my 30th flu season. I would never have expected to see flu activity so low. ”
An online map of influenza activity maintained by the CDC provides striking visual evidence of the effect. In 2020, for the week ending December 19, the map of the United States is a sea of green, showing “minimal” levels in each state. In the corresponding week in 2019, most states were red (“high”) or deep blue (“very high”).
Another virus, enterovirus D68 – linked to a polio-like paralysis that is mainly seen in children – has also been greatly reduced. As recently as August, CDC officials warned doctors and parents to be on the lookout for cases of polio-like condition known as acute slap myelitis. Outbreaks of the condition have peaked in a peculiar cycle every two years for six years with 120 cases across the United States in 2014, 153 in 2016 and 240 in 2018. But by 2020, the CDC confirmed only 29 cases.
“We have not had any positive tests for [enterovirus] D68 … on our side in Colorado since July, and my colleagues around the country also saw very little, ”said Kevin Messacar, a pediatrician of Infectious Diseases at Children’s Hospital Colorado in Aurora. “We have also not seen significant flu or RSV in our wards in Colorado. We do not see all the most common childhood infections that land children in the hospital – flu, ticks, bronchiolitis. ”
As welcome as the absence of these other viruses is during a pandemic, epidemiologists say they see a potentially dangerous consequence after coronavirus cases eventually fall – -one rebound, which could be frighteningly large given the easing of social distancing and reduced immunity to other pathogens.
“The best analogy is a forest fire,” said Bryan Grenfell, an epidemiologist and population biologist at Princeton. “In order for the fire to spread, it must have unburned wood. For epidemics to spread, they require people who have not been infected before. So if people do not get infected this year by these viruses, they will probably at some point later. ”
The possibility of a rebound is not only theoretical: it seems to be happening already in Australia. Official reports showed historically low levels of flu-like illness among children and adults beginning in May, usually the start of the flu season on the hemisphere. The sharp drop in cases came as the country imposed strict closure measures. However, in the last few months, after the coronavirus was almost wiped out and the country ended these restrictions, the number of flu cases among children aged 5 and under began to rise and rise six times before December, when such cases are usually at their lowest.
“It’s an important warning story for us,” Messacar said. “Just because we get through the winter and do not see much RSV or flu does not mean we get out of the woods.”
A breathtaking picture of the extent of the breakdown of viral and bacterial infections can be seen in a single chart created and maintained by BioFire, a company that provides diagnostic tests for viral and bacterial infections to over 2,500 US healthcare providers. The diagram shows the positivity rate for 13 respiratory viruses and bacteria over the last two years.
A year ago at this time, nearly 60 percent of samples taken from patients with flu-like symptoms returned positive for one of the pathogens. But beginning in March of this year, when the coronavirus led shutdowns and school closures, the percentage of samples positive for any other pathogen took a Grand Canyon-worthy jump that dropped about 6 percent in May. Even now, at the time of year when respiratory infections typically begin to peak, only 18 percent of the samples are positive for respiratory viruses or bacteria.
The question, of course, is why SARS-CoV-2, the virus that causes covid-19, continues to spread like wildfire when so many other viruses have been crushed. Epidemiologists say that the primary answer goes back to the “wildfire” metaphor. No one on Earth had been exposed to SARS-CoV-2 before its emergence last year in China, and then we were all fully receptive. (Similarly, the indigenous peoples of North and South America were destroyed when Europeans brought with them not only smallpox, but also chickenpox, cholera, influenza, measles, and other pathogens to which they had never been exposed.)
Viruses that have been circulating for years, on the other hand, are “endemic” or always present at the basic level. Because many of us have previously been exposed to and therefore have developed immunity to them, social distancing can more easily cut the transmission chain.
“A little bit of social distancing can greatly reduce the contagion of common pathogens,” said Lindsay Meyers, who worked for BioFire for 19 years and set up her online cards before leaving the company in the summer. “While a pandemic pathogen, when everyone is susceptible, you need much stricter social distance to contain.”
Epidemiologists use the term “non-pharmaceutical interventions” to describe the range of behavioral changes, such as social distancing., which people use to reduce their risk of infection. In a paper published in the Dec. 1 issue of the Proceedings of the National Academy of Sciences, Grenfell and colleagues noted that such behavioral changes during the 1918 flu pandemic may have lowered measles transmission by 38 percent.
But the focus of the paper was on likely outcome in 2021 and thereafter, after the coronavirus pandemic disappears and such practices are relaxed. Using mathematical models, they focused on the probable effect on respiratory syncytial virus or RSV, a common and highly contagious virus that infects most children before their second birthday and causes over 200,000 admissions and 14,000 deaths in normal years.
“The point is, while we have these controls in place, there is a slow build-up of susceptibility to these other viruses,” said Rachel Baker, a postdoctoral researcher in Grenfell’s department and first author of the newspaper. “The numbers [of infections] was fairly stable for 30 years, but by 2020 it disappeared. We need to be aware that future major outbreaks may be coming. ”
Social distancing is probably not the only factor that suppresses endemic pathogens. Walgreens, for example, has seen “unprecedented demand” for flu shot this season, according to Kevin Ban, medical director of the pharmacy chain.
Unfortunately, vaccines are not available for most other viruses. But another strong factor, called viral interference, could also play an important role in limiting infections. In a paper published Oct. 1 in the Lancet Microbe, Ellen Foxman of Yale University reported that a previous rhinovirus infection greatly reduces the chances of getting the flu A. The reason, she found, is that infection with a virus prompts the body’s immune system to release interferon, which blocks replication of all viruses.
“The interferon response is one of the body’s best defenses against respiratory viruses,” Foxman said. “As soon as it is turned on by a virus, any other virus that comes and tries to grow in the airways cannot.”
In other words, the body’s response to SARS-CoV-2 could block the spread of other viruses. Its response to other pathogens may provide similar protection: Foxman points to temporary pauses in the spread of influenza during the 1918 pandemic as well as the 2009 H1N1 swine flu pandemic, possibly due to viral interference from the rhinovirus.
Virus interference can also help explain a deep mystery about the coronavirus: why so few children become seriously ill from it. As it happens, one of the only endemic viruses still circulating much now is rhinovirus, which typically causes colds. And according to Meyers, “Rhinovirus only circulates in pediatric populations. It has been wiped out in adults. ”
To be clear, no one suggests that people should intentionally infect themselves with rhinovirus in order to protect themselves against SARS-CoV-2.
“I do not advocate for people to infect themselves with anything,” Foxman said bluntly.
But when the pandemic is finally brought under control by the vaccines that are now being rolled out, will it then be safe for most people to shed their face masks and return to “normal” life?
“It’s a real possibility that we’ll have an increased outbreak of the endemic infections,” said Ben Lopman, an epidemiologist at Emory University’s Rollins School of Public Health. “If people have not been infected this year due to actions taken to prevent covid-19, there is a real risk of major outbreaks when we return to normal.”
In addition to a greater than normal number of endemic infections, Lopman added that some of these infections could be more serious than normal again due to declining immunity.
Still, he stressed, “It is certainly a good thing that we are keeping these other infections at bay right now when our hospitals are filled with covid-19 patients. But we learn that the effects of the pandemic are complex. They become long lasting. And we find that they extend beyond just the disease that SARS-CoV-2 causes. ”
It is, of course, an uncertain business to predict the future impact of a pandemic that once in a century.
“This is something that has not happened in our lifetime,” said Messacar, Colorado pediatrician. “We know that there will be more children and adults who are susceptible to enterovirus D-68, influenza, parainfluenza, RSV. But what will actually happen in reality, we do not know. ”
Foxman expressed similar reluctance to predict a fixed premise.
“The drop in respiratory viruses has been really fascinating,” she said. “It’s incredible. We’ve never had a natural experiment like this before. What the consequences will be in the short and long term remains to be seen.”
Only one thing is a security, she added. “Eventually, these viruses will come back. They always find a way. ”