MILWAUKEE — Over the past year, the scientific world has seen a steady flow of research updating what we know about the novel coronavirus, SARS-CoV-2, and how it affects humans. 

Because the virus is so new, researchers are still grappling with many questions about its function. And because of the nature of the scientific process, no single study can completely answer those questions. Instead, new research is constantly challenging our understanding of the pandemic.

Here, we explore some recent studies that have shed new light on the virus.

As the pandemic has stretched into its second year, more tragic losses have piled up around the world. 

According to Johns Hopkins University’s tracker, the official global death count sits at over 3.3 million. The U.S. alone has reported nearly 600,000 lives lost to the virus.

But some believe those huge numbers are not even close to capturing the real toll of COVID-19. In a recent analysis by the University of Washington’s Institute for Health Metrics and Evaluation — which has been a leader in modeling pandemic trends — researchers suggested that the true number of deaths is actually 6.9 million, or more than double the official count.

According to their estimates, almost every country has been undercounting deaths by a wide margin. 

The U.S. has actually seen 900,00 deaths, more than 50% higher than the CDC’s number, their model suggested. And in some countries, including Egypt, Japan and Kazakhstan, the researchers believe the true number of deaths is more than 10 times as high as official reports show.

A lot of factors may play into the gaps — like not enough testing, overwhelmed health systems or only reporting deaths from certain settings like hospitals, the researchers said in a statement.

"The analysis just shows how challenging it has been during the pandemic to accurately track the deaths — and actually, transmission — of COVID,” Dr. Christopher Murray, who heads the Institute for Health Metrics and Evaluation, told NPR. 

The researchers based their estimates on the number of excess deaths, or how many more deaths occurred this year compared to expected numbers. They adjusted their model to account for other pandemic-related factors — like how lack of access to health care could have led to more deaths from other causes, or how an extremely mild flu season brought down this year’s influenza deaths.

Infectious disease expert Dr. Anthony Fauci said in an NBC interview that while he thought the study’s estimates seemed a bit high, there’s “no doubt” we are undercounting the number of COVID-19 deaths.

Since the novel coronavirus first started making its way across the world, it's been hard to get a clear answer on a key question: How exactly does this thing spread?

Last week, the Centers for Disease Control and Prevention updated guidance to reflect that a key way the virus spreads is through tiny particles in the air — ones that can travel beyond the six-foot range of social distancing.

The CDC has long recognized that the virus can spread between people through bigger respiratory droplets, which fall to the floor quickly after a person coughs or sneezes. (Getting infected through fomites, or virus that sticks around on surfaces, is possible, but the risk is low, the CDC says.)

But now, the guidance also advises that people are exposed through “inhalation of very fine respiratory droplets and aerosol particles” — smaller, lighter forms that can stick around in the air for a longer period of time.

The CDC had previously said aerosol transmission was possible, but not a major driver of spread. Now, airborne transmission is at the top of its list of “How COVID-19 Spreads.”

This means that you don’t have to be in “close contact” with a COVID-19 patient to be infected, as the CDC had previously suggested. Especially in crowded indoor settings, these aerosol particles can build up.

“There’s more exposure closer up,” epidemiologist David Michaels told The New York Times. “But when you’re further away, there’s still a risk, and also these particles stay in the air.”

Some experts had been warning since early on in the pandemic that airborne transmission was an important risk during the pandemic. Hundreds of scientists published an open letter last summer calling for more focus on preventing airborne spread, instead of just looking at droplets or surfaces.

The World Health Organization released its own scientific brief last month saying that short-range aerosol transmission “cannot be ruled out.”

As more people start heading back to schools and offices in person, improving ventilation will be key, especially if not everyone is vaccinated, experts said.

“If you’re in a poorly ventilated environment, virus is going to build up in the air, and everyone who’s in that room is going to be exposed,” aerosol expert Linsey Marr told The Times.

After you’re fully vaccinated — as in, two weeks after your last shot — there’s still a chance you could be infected with COVID-19. But that chance is very, very slim, according to recent data.

Last month, the CDC reported around 5,800 “breakthrough cases” among fully vaccinated Americans. Some of the cases were serious, and 74 people died of COVID-19 after getting their shots, according to CNN.

At the time, around 77 million people had gotten all their shots — so the breakthrough cases amounted to around 0.008% of vaccinated people. (These days, more than 117 million Americans are fully vaccinated, the CDC reports.)

Seeing some breakthrough infections is expected, since no vaccine can be foolproof. But the data show that the vaccines are already making a big difference.

"It's a way of taking the pandemic and transforming it from a terrible, ghastly, unmanageable problem, into a manageable problem," Harvard epidemiologist Bill Hanage told NBC News. "It does not reduce the risk to nil, but it does reduce the risk to something that we can handle."

Other data also show that only a tiny proportion of people still catch the coronavirus after vaccination. 

Recent data from the Cleveland Clinic found that between January and April, 99.7% of caregivers who tested positive for COVID-19 had not yet been fully vaccinated. And 99.75% of coronavirus patients admitted to their hospitals were not fully vaccinated. 

In Wisconsin, health officials reported as of April 28 that 605 people — or around 0.03% of fully vaccinated residents — had tested positive. Around half of those with breakthrough infections didn’t report any symptoms, officials told the Associated Press.

These low numbers should be reassuring, and so should the fact that most breakthrough cases are mild or asymptomatic, UW Health’s Dr. Jim Conway told Spectrum News 1.

"[The] intention all along has been to prevent hospitalizations, severe illness and death," Conway said. "And to be honest, these vaccines are actually overachieving and overperforming. Not only that, but the rate of ... breakthrough cases is actually lower than what was seen in the initial studies."

The likelihood of getting infected after being fully vaccinated — which is very small to begin with — also depends on how much virus is moving around in a community. So, as more and more people get their shots, these breakthrough cases should become even more rare, as experts told CNN.

Many COVID-19 patients suffer from lingering symptoms months after they were first infected — and their conditions come in many shapes and sizes, new research confirms.

A large study published in April — thought to be the biggest of its kind — took a look at medical records from 73,000 U.S. patients who had gotten COVID-19 but hadn’t been hospitalized for it, compared to 5 million patients who had never tested positive.

Between one and six months after their infections, the coronavirus patients had a 60% higher risk of dying. And they were 20% more likely to need outpatient medical care, according to the study, which looked at records from the Department of Veterans Affairs health system.

“What we’re seeing in acute COVID, we’re seeing literally the tip of the iceberg,” Dr. Ziyad Al-Aly, one of the study’s authors, told St. Louis Public Radio. “If you start looking at long COVID, that’s what’s beneath that tip.”

Researchers found that patients who had more severe cases at first were more likely to see long-term issues down the line. Still, patients who had mild symptoms early on developed a wide range of problems in the months after they were infected.

Besides commonly reported “long-haul” symptoms like fatigue and brain fog, COVID-19 patients were at higher risk of being newly diagnosed with heart disease, diabetes, anxiety, depression and kidney disease, according to the study. In fact, pretty much every organ in the body could be affected by these post-COVID-19 symptoms, the researchers found.

Scientists still aren’t sure what exactly causes this wide array of symptoms — whether it’s some virus still active in the body, an immune response that’s gone too far or some other factor, as providers told Spectrum News 1. 

But in any case, the authors emphasized, the world should be prepared to deal with the consequences of these cases.

“What we will grapple with for years to come, maybe even for decades, is the effect of the pandemic on the long-term health of Americans,” Al-Aly told The New York Times.

• Some organ transplant recipients may not get good protection from COVID-19 vaccines, a Johns Hopkins study found. Almost half of transplant patients in the study — who have to suppress their immune systems so their bodies don’t reject their new organs — didn’t develop antibodies even after two vaccine doses.

• The Pfizer vaccine got the go-ahead for use in kids aged 12 to 15. But according to a Kaiser Family Foundation survey, under one-third of parents said they’d get their kids in this age group vaccinated right away.

• Studies out of Israel and Qatar found that Pfizer’s vaccine is still very effective against two concerning coronavirus variants: B.1.1.7, first discovered in the U.K., and B.1.351, first discovered in South Africa.