Elimination of COVID-19 – Is This Possible and What Would It Look Like?
An interesting article published in The Lancet Infectious Diseases August 6, 2020 by Anita Heywood and Macintyre looks at this in detail.
In countries that have achieved a low incidence of COVID-19 infection, such as Australia and New Zealand, disease elimination has been proposed. This is in spite of there not being a definition of elimination for COVID-19.
Both countries implemented early, widespread, and strict disease mitigation strategies. However, with low cumulative incidence, most of the population in these countries now remain susceptible to SARS-CoV-2.
Before the availability of a vaccine, implementing exit strategies that ease social distancing restrictions will almost certainly result in epidemics. This is particularly so if a low level of community transmission remains or is imported through travel, as seen with the resurgence in the state of Victoria, Australia in July 2020.
For other respiratory transmitted infections, such as measles, mumps, and smallpox, the pre-vaccine era saw recurrent epidemic cycles, and a similar pattern is projected for unmitigated SARS-CoV-2 transmission, depending on the duration of immunity.
Reduced case counts, flattened epidemic curves, and longer inter-epidemic periods are dependent on achieving immunisation coverage and reduced transmission through implementation of non-pharmaceutical interventions. Immunisation programme outcomes dictate the concepts of disease elimination and eradication.
Disease eradication is the global reduction of infection to zero cases, whereas disease elimination is the absence of sustained endemic community transmission in a country or other geographical region.
With ongoing global SARS-CoV-2 transmission, reduction to zero cases in a defined region is only possible with stringent travel restrictions. It is suggested that the sustained restrictions that reduced travel by 90% to and from Wuhan, China, early in the spread of SARS-CoV-2, only modestly affected the epidemic trajectory to other regions of China.
However, in Australia, travel bans were highly effective in controlling the spread of SARS-CoV-2 into Australia and averted a much larger epidemic.
Following the eradication of smallpox in 1980 the WHO concentrated on the global eradication of polio and the regional elimination of measles as goals. Some countries have achieved the elimination of polio and measles, but complete eradication remains elusive, especially with the global resurgence of measles in 2018 to 2019 following the MMR disputes.
The regional elimination of a pathogen can be achieved through continued control efforts, however eradication of a disease in humans might only be feasible if humans are the only host. This pathogen extinction will also include destruction of all natural and laboratory stored samples. The advances in synthetic biology and genetic engineering make this extinction of any human pathogen very unlikely.
This elimination of any infectious disease is an ambitious strategy to achieve and requires substantial resources. The WHO criteria for elimination of measles was and is evidence of low incidence – high quality surveillance with rapid outbreak response – high population immunity.
Without a vaccine, the criteria of low incidence and high population immunity are mutually exclusive propositions.
The basic reproduction number R0 for COVID-19 probably lies between 2 and 3. Therefore, more than 60% of the population is required to induce herd immunity. Only less than 5% of the population is estimated to have been infected in high burdened countries such as Italy who excluded any consideration of plans to allow unmitigated transmission of COVID-19. In these cases, only vaccination can purposefully achieve a sustained and sufficiently high population immunity to eliminate epidemic respiratory infections such as COVID-19.
Elimination criteria require evidence of the maintenance of R0 below 1 in a health system with the capacity to detect a case of infection if it does occur.
For measles as an example, an adequate surveillance system is measured by an annual rate of two negative measles tests per 100,000 population.
For infections with presymptomatic and asymptomatic transmission, such as COVID-19, several generations of transmission can occur without detection. The ability to detect COVID-19 has been estimated to require weekly COVID-19 testing rates of the population with symptoms of fever and cough of approximately 2000 tests per million population. This is achievable in some countries but this rate of testing may be difficult to maintain in the long term.
In the absence of a WHO goal for COVID-19 elimination, individual countries should instead develop their own criteria for control. This should include extensive surveillance and criteria for differentiating sustained community transmission from sporadic, non-sustained outbreaks.
A low threshold definition of sustained community transmission for COVID-19 could be at least three generations of transmission from an index case or a specified period, such as three months, without new cases. Periods shorter than three months might not be meaningful, and declarations of elimination might result in a false sense of security for the population.
Setting clear parameters for defining resurgence can provide a flag for the start of a potential epidemic period and signal the need for increased use of non-pharmaceutical interventions.
Without country-wide elimination, it is likely that continued management and control of COVID-19 with intermittent periods of restrictions is required until a vaccine is available.