As humans, we fear the unknown, and nothing feels so dangerously mysterious as the novel coronavirus. This week, we are officially two years in to the pandemic. We’ve progressed from panic over what looked like a wave of “atypical pneumonia” cases in China, through global lockdowns, multiple virus variants, and eventual vaccine rollouts. Now, we’re worried about the Omicron variant and it can be easy to forget how much we’ve learned over the past two years. But there are some fundamental truths about this virus — including its known variants — that are worth reiterating.

An awful anniversary

Two years ago this week, hospitals in the city of Wuhan, China, began filling up with people suffering from pneumonia-like symptoms. They had strange spots on their lungs, scans revealed — the“ground glass” opacity characteristic of pneumonia associated with what we now call Covid-19. Reports vary as to whether the first confirmed case occurred in late November, or early December. By December 30, 2019, the Wuhan Municipal Health Commission issued two internal “urgent notices” to the city’s hospitals warning of unusual pneumonia cases seemingly originating from the city’s Huanan Seafood Wholesale Market.

What followed was months and months of uncertainty. Scientists scrambled to learn everything they could about how this novel virus worked while the world watched a massive human tragedy unfold. It was a scary and uncertain time.

We’re now in another period of uncertainty because the virus — SARS-CoV-2 — keeps mutating and changing to become different variants on the original virus. Some notable mentions: Alpha, Beta, Gamma, Delta…and now Omicron.

It is time to revisit nine essential truths that can help keep us safe. These Covid-19 fundamentals may not remain fundamentals given the unpredictable nature of viruses and mutation. But some two years into this horrible pandemic, these nine facts hold up.

These truths also serve as an important reminder that extends beyond the pandemic: Science can help us understand the threats we face, but it’s up to us to pay attention.

9. SARS-CoV-2 is airborne

It was a massive mistake: When Covid-19 first emerged, public-health experts said the virus was transmitted via droplets: large drops that are emitted when someone sneezes or coughs. They claimed we were most likely to catch the virus through close contact with an infected person or via “fomites,” touching objects that an infected person had touched, like a doorknob or elevator button.

People went so far as to wipe their mail down with antibacterial wipes — and yet the virus spread. While hand-washing is always good hygienic advice, we now know the danger lies not on our hands or groceries, but in the air.

By March, experts were sounding the alarm that Covid-19 is actually transmitted through aerosols, microscopic particles emitted when we speak and even breathe.

We now know they were right — and certain new variants like Delta are even more transmissible than past versions of the virus. There’s still much we don’t know about Omicron, but there are reasons to believe it is even more transmissible than Delta.

8. Mask quality matters

A virus being airborne means the quality of one’s mask really matters.

An airborne virus travels via much smaller particles than a virus transmitted via droplets. It’s a bit like the difference between rain and smoke. If you close your doors and windows in a rainstorm, the inside of your house is going to stay dry. If you’re downwind from a wildfire, those closed doors and windows aren’t going to keep all the smoke out of your house, some will slip through cracks.

The same is true of viruses. Airborne viruses can slip through tiny gaps in mask material more easily than droplets. That means masks like N95s or certified KN95s are the safest bet because they are made from a tightly-woven polypropylene material — much tighter than a typical cotton mask.

7. Ventilation is everything

By definition, airborne viral particles hang out in the air for a while — the less airflow, the longer they stick around. Indoors, they can linger in the air for minutes or even hours after an infected person has left the room.

If you’re in a room with people outside your household, get as much ventilation in that room as possible. Opening windows, booting up filters, and turning on fans forces air to circulate, which means any viral particles lingering in the air will be pushed out the window or otherwise disperse.

6. Asymptomatic Covid is common

One reason why Covid-19 is so tricky to quash is that roughly a third of infected people will never know they’re infected. It’s estimated that about 35 percent of novel coronavirus infections are asymptomatic.

If you have no reason to believe you are infected, then you may not be as careful about your interactions with others. In turn, you can inadvertently spread the virus in your community — and you will never know.

This is why many organizations test their employees regularly for Covid-19 regardless of their symptoms. 5. Symptoms take time to develop

People who are symptomatic typically don’t show signs of Covid-19 until at least 2 days after being exposed to the virus; on average it’s 4-5 days post-exposure.

Like people who are asymptomatic, people who don’t yet have symptoms generally have no reason to believe they’ve been infected and so they go about their daily lives

Complicating matters is the time delay between exposure and the earliest a test can possibly detect the virus. The earliest a person can test positive after exposure is 24-48 hours later.

If you’re unvaccinated, the Centers for Disease Control recommend getting tested immediately after exposure to the virus; if that test is negative, they suggest getting tested again in 5-7 days.

4. Most cases of Covid-19 are mild

People who downplay the severity of the virus often say that “most cases of Covid-19 are mild.” That’s accurate. One study suggests 80 percent of Covid-19 infections are mild.

But it’s critically important to define “mild” in the context of Covid-19.

A case of Covid-19 may be called “mild,” according to the same study, if it doesn’t involve pneumonia or involves only mild pneumonia. A high fever, body aches, and a hacking cough that won’t quit can all be symptoms of a mild Covid-19 infection. Further, mild cases of Covid-19 can result in Long Covid.

3. Vaccines offer some protection

Vaccines prompt our immune system to mount a defense against a specific threat, in this case, SARS-CoV-2. They act like a blueprint for our immune system, showing our immune cells what to look for and how to create antibodies that protect us from the worst outcomes. But this is a biological, not magical, process.

The immunity conferred by vaccines isn’t perfect and breakthrough infections do occur. Further, immunity can wane over time.

Right now, we know that vaccines protect the vast majority of vaccinated people from being hospitalized and from dying of Covid-19. We don’t know if Omicron will change the level of protection offered by vaccines, but you’re unquestionably better off vaccinated than not. If you are vaccinated, get a booster.

2. Rapid tests can reduce the spread of Covid-19

The only way to know for sure if you or anyone else has Covid-19 is to get a test. Testing is also a measure of where we are in the pandemic and where it is going. For a clear and reliable measure, we need to test everyone often.

The Biden administration has done more than the previous administration to combat the virus, but testing is one area where the country is still lacking in significant progress. Free and frequent rapid testing is essential to mitigating the spread of Covid-19.

1. This virus mutates

Viruses mutate. The more transmissible a virus is and the more hosts it has, the more it has the opportunity to mutate further. We can’t predict what those mutations will look like or how they will change how the virus behaves. Omicron is new for now, but it will not be the last variant over which we lose sleep.

What we can say is that a version of SARS-CoV-2 will linger for the rest of our lives. That’s not as apocalyptic as it might sound — the flu we contract today is itself a descendent of the Spanish flu, the virus responsible for the 1918 pandemic. Certain mutations may even help us in the long term (for example, mutations that make the virus more transmissible but result in less severe disease, like a common cold).

Good science takes time. After two years, we understand so much more about how this virus works than we did at the beginning, and that alone should give us some comfort.