How long does the virus live on surfaces?

A new study from the Australian Centre for Disease Preparedness has suggested that SARS-CoV-2, the virus that causes COVID-19, can survive on surfaces for up to 28 days. The study, as reported by many media outlets, was worrisome. It implied that the virus could linger on surfaces for much longer than previously thought and meant that even the most meticulous person might come into contact with some virus during normal activities. But an important detail was often omitted in the reporting. Researchers studied the virus under ideal lab conditions, and lab conditions can differ greatly from everyday life.

Most respiratory viruses are spread from person to person via respiratory droplets that are expelled from our nose and mouth every time we talk, laugh, breathe, cough or take it upon ourselves to sing karaoke. In this respect, COVID-19 is no exception. While there remains an ongoing debate about how far these respiratory droplets and aerosols can travel and how long they can linger in the air, there is a general consensus that most new infections of COVID-19 happen directly from one person to another.

There has always been some concern, though, that some new cases of COVID-19 may occur via fomites — inanimate objects or surfaces that can be sources of infection. For example, if you have COVID-19 and cough into your hands, then touch a doorknob, then someone else comes by after you and touches the same doorknob, and then touches their face and introduces the virus into their nasal cavity, that person was infected via fomite transmission.

Fomite transmission obviously happens, as evidenced by the fact that handwashing

Handwashing is one of the most effective ways of stopping disease spread.

is one of the most effective ways of stopping disease spread, especially in hospitals where, unsurprisingly, germs abound. But how much COVID-19 is spread in this way is somewhat unclear.

At the beginning of the pandemic, a research letter in the New England Journal of Medicine suggested that the SARS-CoV-2 virus could survive on surfaces, such as plastics, for up to three days. Even though its survival time was much shorter on other surfaces like copper and cardboard, three days became the standard cut-off for most people when worrying about survival of the virus outside the body.

Which is why this Australian study is so concerning. Researchers have pushed back the potential window for fomite transmission by nearly tenfold. But these types of studies have an important limitation. Lab conditions do not always reflect what happens in everyday life.

In lab studies, researchers will spray a virus-containing solution onto a surface and repeatedly measure for traces of the virus until no more virus can be detected. In these types of controlled situations, viruses would be likely to survive for much longer than they would in the real world. Viruses can't replicate outside a host, and if left on an inert surface, they will simply die off after a while, although how long that takes depends on the conditions in the environment. Factors such as heat and UV light can speed up viral degradation, so researchers tried to control for these factors in their experiment. Viral samples on various surfaces were kept in incubators to keep the samples at a constant temperature. Another unreported detail is that the experiments were also carried out entirely in the dark.

In a real-world situation, the likelihood that they will survive for very long is probably low. Most experts now believe that COVID-19 transmission via contaminated surfaces is probably not a major source of infection.

It's still a good idea to wash your hands, but most infections are probably person-to-person. So the best way to avoid the virus is to keep your distance and wear a mask. That hasn't changed.