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When the World Health Organization declared Covid-19 a global pandemic in March 2020, almost everything about the new coronavirus was an open question: How was it spreading so quickly? How sick would it make people sick? Would a single fight protect you against future cases?
In the four years since then, scientists have unraveled some of the biggest mysteries about Covid. We now know much more about how it spreads (no, being two meters away is not 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’ve learned.
Why do people’s experiences with Covid vary so much? And are super evaders real?
By now, most Americans have had Covid at least once. While most of those infected have shown flu-like symptoms, some have been hospitalized with severe respiratory problems and others have had no symptoms at all.
Part of this can be explained by the amount of virus we are exposed to, but our bodies also play an important role. People who are older or have existing health problems tend to have more severe symptoms because their immune systems are already weakened. In some cases, the body can fight off the virus before it replicates enough to cause symptoms, or clear it so quickly that a person never tests positive. There is also strong evidence that vaccination makes the disease less serious.
Experts said people who have never been infected are most likely to be fully vaccinated, be very cautious about avoiding exposure (by wearing masks and avoiding crowds) or working from home.
Scientists have been trying to investigate whether there is something biologically unique about Covid Overseers That gives them immunity to infections. But the closest they have come is discovering that mutations in the human leukocyte antigen, which tells the immune system that cells are infected, can help clear the virus so quickly that a person could be left completely asymptomatic.
Is the spread of Covid reduced to coughing and sneezing?
In the early days of the pandemic, we all thought Covid was some kind of ninja jumping over the surface. We frantically clean up groceries, wash our hands to the tune of “Happy Birthday,” and try to turn doorknobs with our elbows.
But studies They have since shown that contaminated surfaces are rarely to blame for the spread of the virus. It is more likely to spread through the air we breathe. Part of this may be due to the large droplets produced when someone coughs or sneezes, which is why public health officials advised early in the pandemic that we stay six feet away from other human beings.
But the research He then suggested that the virus could also be carried by aerosols, smaller particles that could infect people from further away. “These particles behave like cigarette smoke: They come out, they float, and they can float in the air for a while,” said Linsey Marr, an environmental engineer at Virginia Tech. Dr. Marr and others have found that tiny particles as small as five microns can transport further infectious virus than larger droplets, in part because they are generated from deeper areas of the lungs.
Other studies have shown that the virus is still evolving to enhance its airborne spread, said Vincent Munster, head of the virus ecology section at the Rocky Mountain Laboratories of the National Institute of Allergy and Infectious Diseases.
How long do our defenses last?
Generally speaking, an infection or a vaccine protects you for several months, said Akiko Iwasaki, a virologist and immunologist at Yale University. But immunity depends on factors such as age, underlying health, and whether the virus has acquired 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 as needed, and T cells that can learn to recognize and predict variations of the virus’s spike protein. virus.
Experts believe that higher levels of antibodies correlate with better protection. But some studies have indicated that antibody levels drop significantly three months after a infection or a vaccination. And it has been a challenge to determine exactly how many antibodies are needed to provide basic protection, “as new variants continually emerge,” Dr. Iwasaki said.
T cells provide a different form of protection (reducing the severity of symptoms rather than blocking infection) and research now suggests that this immunity may last. one year or longer.
What’s behind the strange symptoms?
While a robust immune response is needed to clear the virus, a dysfunctional response may be to blame 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 attaches to ACE2 receptors on cells that support certain nerves in the nose. This triggers a rush of immune cells, which release proteins to clear the infection. In the process, they may inadvertently change gene activity of neighboring nerves, altering the sense of smell.
Since the nose acts as an entry point to the brain and other parts of the central nervous system, this overly aggressive immune response and subsequent inflammation could also be the key to understanding other lingering neurological effects of Covid, such as brain fog, headaches, hums. ears, tingling or numbness in the extremities and even depression, said Dr. María 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 those symptoms have also become less frequent, and past infections or vaccines may have made people’s immune systems less likely to go haywire, Dr. Ruiz said.
Is there such a thing as a Covid seasonal lull?
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 respite. It is true that there are naturally 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 something studies suggest that the virus also remains infectious for longer and that particles carrying it can remain airborne for longer when relative humidity is low.
But Covid does not appear to be inherently seasonal: “we have clearly had surges in the summer as well,” Dr. Marr said.
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, Dr. Munster said.
Do children have a secret weapon that protects them against Covid?
Early in the pandemic, people feared that children, as notorious germ spreaders, would easily contract and spread the virus. They were also concerned about children getting particularly sick, because they tend to experience some of the most severe outcomes with influenza and RSV.
But with Covid, children seem to have largely been spared 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 of why that is: Children’s immune systems may be better prepared against Covid precisely because they are frequently exposed to the benign coronaviruses that cause common colds, said disease specialist Dr. Alpana Waghmare. infectious diseases at Seattle Children’s Hospital. Additionally, studies have shown that another defense mechanism, known as the innate immune response, is stronger in childrenhelping to alert their bodies to foreign pathogens like the virus that causes Covid.
How does the virus wreak havoc on a person for months?
One theory is that, as with other rare side effects, persistent symptoms or new complications that can occur in the months after an initial infection, known as long Covid, are partly due to an immune reaction gone wrong. People who develop long Covid may have an immune system that responds too aggressively, or not aggressively enough, to the acute infection, said Dr. Ziyad Al-Aly, head of research and development at the Health Care System. of St. Louis Veterans Affairs. Studies have also found that the virus can hide in the body once the main infection is over, causing a continuous low-level immune response and inflammation.
Other evidence suggests that the virus can damage the lining of blood vessels, causing small clots that block circulation to various parts of the body. This can cause persistent joint pain, brain fog, chronic fatigue, and dizziness after getting up too suddenly.
Dr Al-Aly said that while many of the mysteries of Covid have been solved, he fears the public has grown tired of the virus, when in reality, he said, “it is not yet in our rearview mirror.”