A year on from the Covid-19 outbreak and the virus has killed almost 1.5 million while infecting more than 65m.
What began as an unexplained pneumonia in patients at hospitals in Wuhan, China, has spread since across the globe, wrecking havoc on economies and devastating industries.
Much is yet unknown about this novel coronavirus but with, scientists around the world working to understand its affects, here are five things we know for sure about Covid-19:
1. Vaccines can prevent people from becoming ill with Covid-19
In just a year, vaccines in clinical trials, shown to be as much as 95 per cent effective at preventing people from falling ill with Covid-19, have been designed, developed, tested and approved.
It represents an impressive scientific achievement given the process typically takes more than a decade and in some cases is ultimately unsuccessful.
Efforts to develop vaccines against some viruses, such as HIV, have failed despite multiple trials. Indeed, when the coronavirus emerged there was no guarantee that vaccine development would yield results.
The speed is thanks to billions of dollars of government funding and because, where possible, the various stages of vaccine development have been run simultaneously instead of sequentially.
What is less clear yet is whether the vaccines being rolled out will stop people spreading the coronavirus as well as preventing them falling ill.
2. Transmission can be airborne
Early on in the pandemic, it was unclear whether airborne transmission of the new coronavirus was possible and greater emphasis was put on the risks from infected surfaces.
It has emerged, however, that virus-containing particles suspended in the air are likely to be a route for transmission.
The journal The Lancet, for example, has said that reports of individuals being infected by people they were not in close contact show airborne transmission can happen.
The risk of airborne transmission means that indoor spaces should be well ventilated to reduce the concentration of virus-containing particles.
Other research published in The Lancet indicates the risk from contaminated surfaces is modest because researchers have struggled to cultivate the virus from objects.
It said surfaces are “unlikely to be a major route of transmission” even though the virus can in some circumstances persist for days outside the body.
3. People not showing symptoms can still spread the coronavirus
About one in five people who contract Covid-19 do not show any symptoms and, while the journal Nature says that any one such individual "will transmit the virus to significantly fewer people than someone with symptoms", they can still pass it on.
One review, reported in Nature, found that asymptomatic individuals were 42 per cent less likely to pass on the virus than people with symptoms. Results vary between studies and one paper suggested a much lower likelihood of transmission in the case of asymptomatic infections.
Other people not showing ill-effects may be in the pre-symptomatic phase before symptoms develop.
Taken together, asymptomatic people and individuals who are pre-symptomatic could account for around half of new coronavirus cases, according to one study.
Some research indicates that people with more severe symptoms produce greater levels antibodies against the virus and their immune response may be longer lasting, potentially offering stronger protection against reinfection.
4. Some infected people are much more likely than others to spread the disease
People infected with Covid-19 vary dramatically in how infectious they are, with some individuals much more likely pass the pathogen on.
Branded superspreaders, they make up between about 10 to 20 per cent of those infected, according to reports, but could be responsible for as many as four in five new cases.
Factors such as breathing more quickly or talking loudly are thought to make a person more likely to spread the virus.
However, even when talking in a similar fashion, individuals vary widely in how many potentially virus-containing particles they emit.
Research done prior to the pandemic has said that some people are “super emitters” of respiratory droplets, perhaps because of the consistency of fluids in the respiratory system.
When these are thicker or more viscous, a person is more likely to emit tiny particles called aerosols, which can linger in the air for long periods.
5. Wearing a face mask reduces transmission
The effectiveness of face masks has been much discussed, but there are multiple pieces of evidence that they can reduce transmission.
They are thought to be particularly effective when people are asymptomatic or pre-symptomatic and so may be unaware they are infected. Multi-layered masks without valves are recommended.
Studies in Canada, Germany and the US, for example, have reported significant reductions in infection rates after people were told to wear masks.
Other observational studies in environments ranging from warships to hair salons, aircraft and households have also found reductions in risk of up to 70 percent associated with wearing masks.
The US Centres for Disease Control (CDC) has said mask use offers benefits both to the mask wearer (by filtering out fine particles containing the virus that a person may inhale) and to others (by reducing the number of virus-containing particles an individual releases).
Studies also indicate that even if infection happens, masks may reduce the severity of illness because the wearer has been exposed to fewer virus particles.
