Covid-19: If you're the only one in a mask, will it still keep you safe from infection?

Masks were once mandatory in public places but, as the rules have loosened, fewer people appear to choose to wear one.

So what does that mean for people who are still masking up? Are you still protected if others aren't wearing one?

Here's what you need to know:

Do I still have to wear a mask?

In New Zealand, the government has dropped most mask requirements but masks are still mandated when you visit most healthcare services, such as:

• doctors' clinics
• community and iwi health providers
• pharmacies - but not those inside supermarkets
• hospitals
• urgent care services, such as after-hours clinics
• dentists and oral health services
• disability support services
• residential care for older people and people with disabilities
• other health services such as optometrists and physiotherapists.

For patients and workers at healthcare services, mask requirements are decided locally.

But the Covid-19 website stated people are still encouraged to wear a mask in crowded places, such as public transport, enclosed spaces with poor ventilation, close contact settings, such as face-to-face conversations, or when visiting people at high risk.

It is also recommended for household contacts of confirmed Covid-19 cases, those testing daily for five days, and people at higher risk of getting seriously ill from Covid-19 or keen to reduce risk of becoming sick.

"Face masks remain an important defence against Covid-19," the website states.

Will wearing a mask stop you getting Covid-19 if no-one else is wearing one?

Over in Australia, Monash University's school of public health head of epidemiological modelling James Trauer told the ABC that wearing a mask would provide some protection, but was unlikely to be completely effective.

"It is also quite dependent on how the mask is worn and many other factors, such as the amount of Covid around in the community and the setting you visit," Prof Trauer said.

Wearing a mask was still considerably better than nothing, he said.

"However, it is likely that the greatest effect is when the infector [the person with Covid-19] is wearing one," he said.

"This is because wearing any sort of mask should markedly reduce the number of larger droplets they expel.

"This is one of the strongest justifications for requiring everyone in the population to wear a mask - particularly indoors where most transmission occurs."

In the Australian state of Victoria, multiple investigators were able to show a considerable reduction in transmission when the state mandated face coverings in most settings in July 2020, Prof Trauer said.

"We likely saw that effect because even people who didn't have symptoms and so didn't know they had Covid were still wearing masks and so reducing their risk of transmitting infection to others," he said.

"A review of population-wide studies found that if everyone in the population wears a mask, the overall infectiousness of Covid can be reduced by about 15-20 percent."

Deakin University professor and chair in epidemiology at the school of health and social development Dr Catherine Bennett agreed there was evidence which proved masks reduced Covid-19 transmission rates.

"A US study of close contacts of cases from a year ago found the infection rate in contacts was about 26 percent if the infected person, their contact, or both were unmasked, but dropped to about 13 percent if both were masked," she said.

"However, a large population-based study in the UK when Omicron had taken over [in February 2022] did not find a difference in infection rates based on whether masks were worn indoors always, or less frequently.

"But this can also be complicated as those wearing masks might do that because they are at higher risk anyway, and might have had even higher rates of infection without them.

"So the advice remains that wearing masks is still a useful way to reduce your risk of picking up or spreading the virus."

What's the science of mask-wearing?

Covid-19 is primarily transmitted by the respiratory route, meaning that breathing, coughing, speaking, laughing and singing could create particles that contained the virus, Prof Trauer said.

"We divide respiratory transmission into transmission with larger and smaller particles.

"Larger particles are called droplets, whereas smaller ones (called aerosols) can travel longer distances because they can be suspended in air.

"In reality, there is no definite cut-off between these two processes, and both are likely important.

"In 2020, many policy makers and experts were too slow to recognise the importance of the smaller particles.

"We now have several lines of evidence to indicate that the smaller particles are important, even though it's impossible to define this exactly."

Dr Bennett said the finer the mask fibre and the better the fit, the better the mask worked.

"This is why N95 mask are recommended as they can even filter very small aerosol particles," she said.

"Any mask that doesn't fit properly doesn't offer as good level of protection, the air can still pass between the mask and your skin, and fewer virus particles will be filtered out."

What are the different types of masks and which are the best ones?

Dr Bennett said N95 or P2 masks are the best protection if worn correctly, however they could be hard to fit and less comfortable.

"Surgical masks are the next preference, but again, the less gap around the edges, the better," she said.

"Cloth masks are the least effective, but better than nothing, and one of these that fits your face well over a surgical mask can help the surgical mask to work better."

However, Professor Trauer said no mask was completely effective protection because breathing was not the only means of transmission.

"It is probably possible to transmit the Covid virus through objects ("fomites") and other mucosal surfaces (such as the surface of the eyes)," he said.

"To get close to fully protected, you would ideally wear eye protection, a respiratory mask, be careful with hand hygiene and avoid high-risk settings."

What are the experts doing with their own masks?

Both experts spoken to by ABC still wear a mask where required.

"I am now only wearing a mask in healthcare and aged care settings, for the reasons given above," Prof Trauer said.

"I kept my mask wearing up a bit longer than most because I wanted to make sure I was practising what I was preaching."

Dr Bennett said she still wore N95 masks for more protracted exposure times such as on planes or public transport, and wore a surgical mask in busy indoors settings.

"I'll often wear the mask as I walk into a shop or venue then decide whether I need to keep it on depending on the number of people, the size of place and ceiling height, and the airflow," Dr Bennett said.

"If small and stuffy, I'll keep my mask on even if there are not many people around.

"I haven't had Covid, so it's working for me so far."

-ABC, with additional reporting by RNZ