ince the Covid-19 outbreak began, many have been concerned about asymptomatic spread – that is, people who have the virus but show no symptoms, so therefore don’t take measures to quarantine themselves. As the virus spreads throughout the world more research can be gathered, and scientists are learning more about asymptomatic spread and its prevalence. Findings published in the BMJ Journal Thorax on Thursday found asymptomatic cases of Covid-19 meant the prevalence of the virus was likely to be significantly underestimated on cruise ships. Australian researchers led by Prof Alvin Ing from Macquarie University in Sydney analysed tests from all 217 passengers and crew on board a ship that departed from Argentina for a 21-day cruise of the Antarctic Peninsula in mid-March, after the World Health Organisation declared a pandemic. They found more than 80% of the 128 people who tested positive for Covid-19 had no symptoms. In a separate study published on Thursday in the Journal of the American Medical Association (JAMA), 42% of 78 close contacts of Covid-19 patients in a hospital in Wuhan, China, showed no symptoms of the disease. The researchers found the asymptomatic study participants had a shorter duration of viral shedding, a shorter duration of disease and were less likely to pass it on. The asymptomatic cases were more likely to be women, and to only shed the virus (shedding refers to the replication of the virus inside the body being released into the environment, such as on to surfaces or into the air) for eight days compared with 19 for those with symptoms. Though they display less viral shedding, asymptomatic people can still spread disease. How common is asymptomatic spread then? We still don’t know. There have been varying studies, but some of them have been conducted in unique environments such as hospitals or aged care homes, where the demographics of the populations may be different. It is also possible that some people who were asymptomatic at the time of testing later went on to develop symptoms. Sanjaya Senanayake, an associate professor of medicine at the Australian National University, said about 40% of people were asymptomatic in a study from Iceland, while 30.8% of people in a study of Japanese evacuees from Wuhan were asymptomatic. “So it’s hard to know which is right,” he said. “And although we are getting closer to understanding the proportion of asymptomatic cases with Covid-19, we still don’t know for sure the magnitude of the impact that they have on further transmission of cases. Do they generate lots of secondary cases or only a small proportion?” Prof Ivo Mueller, an epidemiologist at the Walter and Eliza Hall Institute of Medical Research in Melbourne, said it was important to remember that a cruise ship was a contained, unique environment and the percentage of asymptomatic cases found on them would not necessarily translate into the same rates in the community. He said the new research was important, but was not enough to definitely state how common asymptomatic infections of Covid-19 are, or how much these contributed to transmission. “The answers to these questions are not only essential for getting a better picture of how many people have already been exposed, but will also greatly affect our predictions on how the Covid-19 pandemic may develop over the coming months, and what interventions are most important to prevent second wave of cases and deaths,” he said. How do asymptomatic people spread the virus? Infectious disease doctor Peter Collignon said speaking, shouting and singing were all ways someone with no symptoms could spread the virus. “Touching inside the nose and then touching surfaces is another way,” he said. There are many diseases where people can be asymptomatic. Influenza is one of them, typhoid another, as the now infamous case of “Typhoid Mary”, who despite showing no symptoms spread the disease to more than 500 people in the US, shows. This is why it is important to identify asymptomatic cases, even if those infected feel fine. “Although patients who were asymptomatic experienced less harm to themselves, they may have been unaware of their disease and therefore not isolated themselves or sought treatment, or they may have been overlooked by healthcare workers and thus unknowingly transmitted the virus to others,” the JAMA study found. How worried should we be in Australia about asymptomatic spread? It is reassuring that the people who are asymptomatic and have the virus don’t seem to get worse, said Prof Bruce Thompson, dean of the school of health sciences at Swinburne University. Less severe disease and therefore less coughing and vomiting meant there was less opportunity for it to contaminate surfaces and infect people. “We haven’t seen deaths in the population of people who were positive but asymptomatic,” he said. “They also didn’t transmit or shed the virus for as long and finally, recovered quicker. This research highlights the importance of continuing to do more testing.” Prof Raina MacIntyre, head of the biosecurity program at the Kirby Institute, said studies in aged care and other outbreaks had found 50% or more of all positive cases are asymptomatic. “High-risk contacts in outbreak situations, whether family contacts or in a closed setting outbreak, should be tested regardless of symptoms or cases will be missed,” she said. Maintaining good hygiene, following guidelines on social-distancing, self-isolating if you come into contact with someone who tested positive with the virus, even if they are asymptomatic, and self-isolating even if you test positive but are asymptomatic are all strong ways to prevent spread.