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 did it spread so quickly? How sick would it make people? Would a single bout buy you protection against future cases?
In the four years since, scientists have unraveled some of the biggest mysteries about Covid. Now we know a lot more about how it spreads (no, standing 6 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 are superdodgers real?
By now, most Americans have contracted Covid at least once. While the majority of people infected have been struck down with 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 a big 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 the virus before it multiplies 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 severe.
Experts said that most likely, people who have never been infected are fully vaccinated, very careful about avoiding exposure (by covering up and avoiding crowding) or working from home.
Scientists are trying to investigate whether there is something biologically unique about the Covid superhumans that gives them immunity to infection. But the closest they’ve come is that mutations in the human leukocyte antigen – which signals to the immune system that the cells are infected – can help clear the virus so quickly that a person can be completely asymptomatic.
Is the spread of Covid due to coughing and sneezing?
In the early days of the pandemic, we all thought that Covid was some kind of surface jumping ninja. We frantically swept up the groceries, washed our hands to the beat of “Happy Birthday” and tried to turn doorknobs with our elbows.
However, studies have since shown that contaminated surfaces are rarely responsible for the spread of the virus. It is more likely to spread through the air we breathe. Some of that may be due to large droplets produced when someone coughs or sneezes, which is why public health officials advised early in the pandemic to stay six feet away from other people.
But research then suggested the virus could also be carried by aerosols, smaller particles that could infect people from further away. “These particles behave kind of like cigarette smoke — they come out and float and can get carried away 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 carry more infectious virus than larger droplets, in part because they are generated from deeper in the lungs.
Other studies have shown that the virus is still evolving to become better at spreading through the air, said Vincent Munster, head of the division of viral ecology at the National Institute of Allergy and Infectious Diseases’ Rocky Mountain Laboratory.
How long does our defense last?
In general, an infection or vaccination 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 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 make more antibodies as needed, and T cells that can learn to recognize and predict variations in the virus’s spike protein.
Experts believe that higher antibody levels correlate with better protection. However, some studies have shown that antibody levels drop significantly three months after an infection or vaccination. And it has been difficult to determine exactly how many antibodies are needed to provide basic protection, “as new variants are emerging all the time,” Dr. Iwasaki said.
T cells provide a different form of protection—reducing the severity of symptoms rather than blocking infection—and research now shows that this immunity can last a year or more.
What is behind the strange symptoms?
While a strong immune response is required to eliminate the virus, a dysfunctional one can account for many of the unusual side effects of Covid. For example, researchers have found that in people who develop a distorted sense of smell or lose it altogether, 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. Along the way, they can inadvertently change the genetic activity of nearby nerves, disrupting the sense of smell.
Since the nose acts as an entry point to the brain and other parts of the central nervous system, this over-aggressive immune response and subsequent inflammation could also be key to understanding other lingering neurological effects of Covid, such as brain fog , headaches, ringing. ears, tingling or numbness in the extremities, even depression, said Dr. Maria Elena Ruiz, an infectious disease specialist at George Washington University.
The painful swelling or discoloration that some people develop on their fingers or toes remains more mysterious. But reports of these symptoms have also become less frequent, and it’s possible that earlier infections or vaccinations have made people’s immune systems less likely to break down, Dr. Ruiz said.
Is there such a thing as a seasonal break from 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 respite. 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 some studies suggest that the virus also remains infectious longer, and the particles that carry it can stay in the air longer when the relative humidity is low.
But Covid doesn’t appear to be inherently seasonal – “we’ve clearly had spikes in the summer as well,” Dr Marr said.
But experts agreed they would not be surprised if Covid eventually settles into a predictable seasonal pattern, like other respiratory viruses. It’s just hard to predict whether this will last a few years or even decades, Dr. Munster said.
Do children have a secret weapon that protects them from Covid?
At the beginning of the pandemic, people feared that children, as notorious carriers of germs, would catch and spread the virus easily. They also worried that children would get especially sick because they tend to experience some of the more severe effects with flu and RSV
But with Covid, children seem to have largely escaped 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 this is happening: 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 Dr. Alpana Waghmare, an infectious disease specialist at Seattle Children’s Hospital. In addition, studies have shown that another defense mechanism, known as the innate immune response, is stronger in children, helping their bodies to alert them to foreign pathogens such as 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, the lingering symptoms or new complications that can appear in the months after an initial infection – known as long Covid – are caused in part by an immune response gone wrong. People who develop long-term Covid may have immune systems that respond too aggressively or not aggressively enough to the acute infection, said Dr. Ziyad Al-Aly, chief of research and development at the St. Louis Veterans Health System. Studies have also found that the virus can hide in the body after the primary infection is over, causing an ongoing, low-level immune response and inflammation.
Other evidence shows that the virus can damage the lining of blood vessels, causing tiny clots that block circulation to various 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 the mysteries of Covid have been solved, he fears the public has grown weary of the virus – when in reality, he said, “it is not yet in our mirror”.
