September 3, 2020
Confused by some of the terminology you are hearing in all the COVID-19 updates by politicians and medical officers?
University of NSW infectious disease experts have tried to explain what some of the terms mean in their precise medical definition rather than in popular understanding.
Disease management terms such as “elimination” and “eradication” have been used in press conferences and media coverage since the start of the pandemic.
Associate Professor Anita Heywood said while these terms seem familiar, they are technical public health terms which mean something very specific in an infectious disease context – and misuse of the terms can be at best confusing, or at worst, misleading.
“A lot of the confusion comes from these terms being thrown around – sometimes interchangeably,” she said.
“Eradication means no global cases of a disease, except samples in laboratories. We achieved this for smallpox in 1980.
“Elimination, on the other hand, means no sustained community transmission in a specific region or country.”
Assoc Prof Heywood said eradication was not likely for COVID-19 given that there isn’t a vaccine yet and some infected people are asymptomatic (ie. have no symptoms).
The fact that the disease is also “zoonotic” (ie. originating from animals) also makes eradication unlikely. Eradication might only be possible if humans were the only host.
However, Assoc Prof Heywood said the concept of “elimination” shouldn’t be easily dismissed.
“When most people hear the term ‘elimination’, they think zero cases,” she said.
“But occasional cases and small outbreaks can still happen once a disease is eliminated, due to imported cases.
“The key feature of elimination is stopping these cases from leading to sustained community transmission.”
Australia’s official COVID-19 strategy, “suppression”, doesn’t have an epidemiological definition.
“Suppression isn’t a known technical term,” epidemiologist Prof Raina MacIntyre, head of the Biosecurity Program at the Kirby Institute at UNSW, said.
“It’s either elimination or disease control.”
She said “disease control” meant reducing the number of cases to a locally acceptable level, but community transmission may still occur.
“Suppression” could be classified as “disease control” but the unclear definition had implications for the type of goals set, such as what a locally acceptable level of COVID-19 was.
“You need a technical definition so that people know what they’re working towards,” Prof MacIntyre said.
“You also need specific goals and ways to measure those goals.
“For example, when we have achieved zero community transmission, when do we say we have eliminated COVID-19? We need an agreed definition – either a specified time period or a number of generations of transmission.
“Declaring elimination after one month of zero community transmission is meaningless and may lead to complacency with all the disease control measures we still need. The majority of Australians remain non-immune to SARS-COV-2, so we may see more epidemics until the time we can vaccinate most Australians.”
Prof MacIntyre said that when people say “suppression”, they often include the concept that we’ll live with a certain level of disease.
“But COVID-19 isn’t a chronic disease that will remain at the same level. It’s an epidemic disease, which means if you’ve got a certain level of disease, it’s just going to keep growing and getting bigger,” she said.
The rate of infection – or reproduction number called “R” – is the average amount of people someone with COVID-19 is likely to spread the disease to.
R figures greater than one can quickly lead to exponential growth.
“Saying we accept a certain level of disease is a huge risk,” Prof MacIntyre said.
“That’s exactly what we saw in Melbourne. In a matter of weeks, it went from around 15 cases a day to over 700 cases a day.
“COVID-19 infections won’t stay at small levels without strict measures, like the current lockdowns in Melbourne.”
Given there is no COVID-19 vaccine yet, non-medical public health measures – called non-pharmaceutical interventions or NPIs – are being used to manage the disease.
Australia’s current disease control measures include stopping clusters from spreading – such as isolating active cases, conducting contact tracing, and testing and quarantining contacts of known cases.
It also includes preventative measures, such as enforcing border controls, implementing physical distancing requirements (such as lockdowns and mass gathering restrictions), and reducing transmission via face masks.
“The measures they’re using to suppress are the same as what you would use to eliminate,” Assoc Prof Heywood said.
“It’s just how much pressure – the strictness and duration – is put on those measures.”
To make matters even more complex, the definition of “elimination” isn’t static, but changes according to the specific disease and how it spreads.
For example, for measles to be considered eliminated, there needs to be no evidence of community transmission for three years. After the three years, there can be outbreaks, but they can’t last for longer than a year.
Other diseases have their own elimination requirements, which largely vary by the amount of time needed with no evidence of community transmission.
Given these varying definitions, it’s important to be specific when speaking about COVID-19 elimination.
The problem is a definition of COVID-19 elimination doesn’t exist yet and the World Health Organisation has not set a goal.
Assoc Prof Heywood and Prof MacIntyre have suggested a possible definition of at least three months without any new cases of community transmission or at least three generations of transmission from an index case.
“Periods shorter than three months might not be meaningful, and declarations of elimination might result in a false sense of security for the population,” they said.
Assoc Prof Heywood said occasional cases don’t mean elimination is impossible – the key is to stop them leading to sustained community transmission.
“Unless we have a 100 per cent infection control practices for return travellers, there’s always going to be a chance of the virus escaping,” she said.
“The recent spike in New Zealand can teach us the need to stay vigilant.”
Prof MacIntyre said elimination would be a bigger challenge for places with a high number of international arrivals, such as Sydney and Melbourne.
Other parts of the country were in a better position.
“It’s feasible for some States that have no known cases to keep it like that,” Prof MacIntyre said. “But they have to keep their borders shut or tightly controlled with quarantine.
“No matter the strategy, we will be walking the COVID-19 tightrope of waxing and waning epidemics until we have an effective vaccine.
“Everyone needs to play their part in stopping the disease spread – social distancing, wearing a mask, hand washing and getting tested if required.”