Four years later, the mysteries of Covid are unraveling
When the World Health Organization declared Covid-19 a global pandemic in March 2020, almost everything about the novel coronavirus was an open question: How did it spread so quickly? How sick would it make people? Would a single attack protect you from future cases?
In the four years since, scientists have unraveled some of the biggest mysteries about Covid. We now know a lot more about how it spreads (no, standing six feet apart isn’t foolproof protection), why it doesn’t seem to make children as sick as adults, and what’s behind the strange symptoms it can cause. from brain fog to “Covid toe.” Here’s a look at what we learned.
Why do people’s experiences with Covid vary so much? And do super dodgers really exist?
By now, most Americans have had Covid at least once. While the majority of those infected have suffered flu-like symptoms, some have been hospitalized with severe breathing problems and others have had no symptoms at all.
Some of this can be explained by the amount of virus we are exposed to, but our bodies also play a big role. People who are older or have existing health problems tend to have more severe symptoms because their immune systems already have them impaired. In some cases, the body can fight the virus before it multiplies enough to cause symptoms, or clear it so quickly that a person never tests positive. There are also strong indications for this vaccination makes the disease less serious.
Experts said people who have never been infected are most likely fully vaccinated, being very careful about avoiding exposure (by masking up and avoiding crowds) or working from home.
Scientists have tried to investigate whether something biologically unique is going on Covid super dodgers that gives them immunity against infections. But the closest they’ve come is discovering that mutations in the human leukocyte antigen – which signals to the immune system that cells are infected – can help clear the virus so quickly that a person could be completely asymptomatic.
Does the spread of Covid come down to coughing and sneezing?
In the early days of the pandemic, we all thought Covid was some kind of shallow ninja. We frantically wiped down the groceries, washed our hands to the tune of “Happy Birthday,” and tried to turn the doorknobs with our elbows.
But studies have already demonstrated this contaminated surfaces are rare to blame for the spread of the virus. It is more likely to spread through the air we breathe. Some of this may be due to large droplets produced when someone coughs or sneezes. That’s why public health officials recommended early in the pandemic that we stay six feet away from fellow humans.
But research then suggested that the virus could also be transmitted by aerosols, smaller particles that could infect people from further away. “These particles act a bit like cigarette smoke: they come out and float around, and can stay in the air for a while,” said Linsey Marr, an environmental engineer at Virginia Tech. Dr. Marr and others have discovered that it can transport small particles as small as five microns more contagious virus than larger droplets, partly because they are generated deeper in the lungs.
Other studies have shown that the virus is still evolving to spread better through the air, said Vincent Munster, chief of the virus ecology division at the Rocky Mountain Laboratories of the National Institute of Allergy and Infectious Diseases.
How long will our defense last?
In general, an infection or vaccination will protect you for several months, says Akiko Iwasaki, a virologist and immunologist at Yale University. But immunity depends on factors such as age, underlying health and whether the virus has picked up mutations that help it evade our defenses.
There are many components of immune protection, including antibodies that circulate in the blood and help detect and neutralize the virus, B cells that produce more antibodies when necessary, and T cells that can learn to recognize and predict variations of the virus spike protein.
Experts believe that higher antibody levels correlate with better protection. But some studies have shown that antibody levels drop significantly three months after an illness infection or a vaccination. And it has been challenging to determine exactly how many antibodies are needed to provide basic protection, “as new variants are emerging all the time,” said Dr. Iwasaki.
T cells provide a different form of protection – reducing the severity of symptoms rather than blocking infections – and research now suggests this immunity may last a year or longer.
What’s behind the strange symptoms?
While a robust immune response is necessary to eliminate the virus, a dysfunctional immune response may be responsible for many of Covid’s unusual side effects. For example, researchers have found that in people who develop a distorted sense of smell or lose it completely, the virus latches onto ACE2 receptors in cells that support certain nerves in the nose. This triggers a rush of immune cells, which release proteins to clear the infection. They can do so unintentionally change genetic activity of neighboring nerves, disrupting the sense of smell.
Because the nose acts as a gateway to the brain and other parts of the central nervous system, this overly aggressive immune response and resulting inflammation could also be key to understanding other lingering neurological effects of Covid, such as brain fog, headaches, the ears, tingling or numbness in the extremities and even depression, said Dr. Maria Elena Ruiz, an infectious disease specialist at George Washington University.
The painful swelling or discoloration that some people develop in their fingers or toes remains more mysterious. But reports of these symptoms are also less common, and it’s possible that previous infections or vaccinations have made it less likely for people’s immune systems to go haywire, said Dr. Ruiz.
Is there such a thing as a season break due to Covid?
When Covid first took off in the winter of 2020, many people hoped that the summer months (at least in some parts of the world) would bring a reprieve. It is true that there are of course more opportunities for aerosol transmission of Covid in the colder months, when people spend more time indoors. Buildings are also more closed in winter, leading to poorer ventilation and potentially higher levels of airborne pathogens. And a little studies suggest that the virus also remains contagious for longer and that particles carrying the virus can remain in the air for longer when the relative humidity is low.
But Covid doesn’t appear to be inherently seasonal – “we’ve clearly had peaks in the summer as well,” said Dr. Marr.
But experts agreed they wouldn’t be surprised if Covid eventually settled into a predictable seasonal pattern, like other respiratory viruses. It’s just hard to predict whether that will take a few more years or even decades, said Dr. Munster.
Do children have a secret weapon that protects them from Covid?
Early in the pandemic, people feared that children, as notorious disease vectors, could easily contract and spread the virus. They also worried that children would become particularly ill, as they suffer the most severe effects from influenza and RSV
But thanks to Covid, children appear to have been largely spared from serious illness. Only a small number are hospitalized or develop life-threatening conditions such as multisystem inflammatory syndrome, or MIS-C.
We now have a clearer idea why that’s the case: Children’s immune systems may be better prepared for Covid precisely because they are often exposed to the benign coronaviruses that cause the common cold, said Dr. Alpana Waghmare, an infectious disease specialist at Seattle Children’s Hospital. In addition, studies have shown that there is another defense mechanism known as the innate immune response stronger in childrenalerting their bodies to foreign pathogens, such as the virus that causes Covid.
How can the virus cause damage to a person for months?
One theory is that, as with other rare side effects, the persistent symptoms or new complications that can arise in the months after an initial infection – known as long Covid – are partly caused by a stalled immune response. People who develop a long bout of Covid-19 may have an immune system that responds too aggressively or not aggressively enough to acute infections, said Dr. Ziyad Al-Aly, chief research and development officer at the Veterans Affairs St. Louis Healthcare System. Research has also shown that the virus can do that hide in the body after the main infection is over, causing an ongoing, low immune response and inflammation.
Other evidence suggests that the virus can damage the lining of blood vessels, causing small clots to form that block circulation to different parts of the body. This can cause persistent joint pain, brain fog, chronic fatigue, and dizziness after standing up too suddenly.
Dr. Al-Aly said that while many of Covid’s mysteries have been solved, he fears the public has grown tired of the virus – when in reality, he said, it is “not yet in our rear-view mirror.”