With the world at a standstill as countries work hard to manage the current COVID-19 pandemic, it’s hard not to wonder what made this outbreak particularly serious. We spoke to Dr Maryza Graham, Medical Microbiologist and Infectious Diseases Physician at Monash Health, to find out how COVID-19 differs from other respiratory illnesses such as influenza, and why this outbreak has triggered such a major global response.
“Symptomatic infection of COVID-19 can range from mild to critical. Pneumonia appears to be the most frequent serious result of infection and is characterised primarily by fever, cough, shortness of breath, and also by changes on X-rays or CT scans of the chest. These cases can range from very serious, with the need for intensive care admission and ventilation, to mild cases of pneumonia which can often be treated with observation in hospital, with or without small amounts of oxygen.
“Other respiratory viruses, including influenza, can cause a similar spectrum of pneumonia, from very mild to very severe. However, the proportion of people with severe disease appears to be different for COVID-19: current data suggests that around 80% of infections are mild or asymptomatic, 15% are severe, requiring oxygen and 5% are critical infections, requiring ventilation. These fractions of severe and critical infection seem to be higher than what is observed for seasonal influenza.
“What we have also found is that severe pneumonia from COVID-19 occurs predominantly in older adults or in those with underlying medical conditions, and far less commonly in otherwise healthy individuals of any age. With influenza, children are important drivers of transmission in the community. Initial data for COVID-19 suggests that children are less affected than adults, and clinical attack rates in the 0-19 age group are low,” said Dr Graham.
Whilst influenza has an incubation period (the time from infection to the appearance of symptoms) of 1-4 days, the World Health Organisation (WHO) currently estimates that the incubation period for COVID-19 ranges from 1 to 14 days, with a median incubation period of 5 to 6 days. Some of those who are infected may be contagious in the 1-3 days before they develop symptoms, however it is not currently known how often that happens. COVID-19 is mainly spread through respiratory droplets from people who are coughing or have other symptoms such as fever.
A reproduction number (Ro) is often used to reflect how infectious a disease is by measuring the average number of secondary infections caused by a single case. The WHO currently estimates the Ro for COVID-19 to be between 2 and 2.5, which is higher than for influenza during an average season. However, estimates for both COVID-19 and influenza viruses are very context and time-specific which makes direct comparisons more difficult. There has also been a large variation in Ro for COVID-19 calculated by different studies[1].
“Another difference we see is that the mortality rate for COVID-19 appears higher than for influenza, especially seasonal influenza. Based on current estimates, it is thought that approximately 1% of COVID-19 patients will die, although the true mortality will take some time to fully elucidate as it is still very early in the pandemic. By comparison, the mortality rate for influenza is usually well below 0.1%. Whilst this is an important comparison, it is important to bear in mind that mortality is, to a large extent, determined by access to and quality of health care.
“As with very severe pneumonia caused by other infections, lung fibrosis or scarring may form during the healing process, but it is currently too early to know how long these changes will last in individual patients with severe COVID-19 or how permanent those changes could be.
“Currently, Australia does not have widespread community transmission of COVID-19 but continued vigilance is required as we learn more about this new infection,” said Dr Graham.
