By modeling the effect of different vaccination scenarios, researchers say rapid mass immunization will terminate the disease, or immunizing 66% of the population with a 90% efficacy vaccine will achieve herd immunity.
The approval of some vaccines recently against COVID-19, the pandemic disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), marks a turning point in the fight against the disease. The different vaccines have reported efficacies ranging from over 90% for mRNA vaccines to about 62% for the ChAdOx1 two-dose version.
Several more in different phases of the clinical trial will possibly be approved for use in 2021. However, there will likely be a shortage of vaccines in the early stages, and there will need to be a strategy to effectively utilize vaccines in the initial phase and plans for expanded vaccination later on.
Some strategies call for vaccinating healthcare and aged care workers, frontline disease responders, older adults, and people with a higher risk for the disease. However, prioritizing young adults may affect transmission more as the transmission is highest in this demographic. Another could be a ring vaccination strategy, where closed contacts of a confirmed case are identified and vaccinated.
Targeted vaccination strategies with a restricted supply for 1 million people: From left to right is the epidemic curve, the cumulative case and deaths numbers by targeted age group vaccinated. Image Credit: https://www.medrxiv.org/content/10.1101/2020.12.15.20248278v2.full.pdf
Effect on virus spread under different vaccination scenarios
In Australia, the priority groups for early COVID-19 vaccination start with people with a high risk of severe disease such as older adults, Aboriginals and Torres Strait Islander people, and high-risk conditions. The next group is healthcare workers, and the third priority group is people working in critical services.
In a new study published in the preprint server medRxiv*, researchers from the University of New South Wales in Australia have reported the impact of the different vaccine strategies under limited vaccine availability for New South Wales using a model.
They modified a previous model for COVID-19 spread, which moves the population into 16 compartments defining various conditions of the disease, such as not infectious, pre-symptomatic infectious and diagnosed, symptomatic, recovered, vaccinated, and the like. To test the most effective vaccination strategy, the authors used a hypothetical scenario with 100 symptomatic and 250 untraced latent infected people as a start.
The vaccine strategies they tested were limited vaccine supply, limited vaccine supply, and ring vaccination, and unlimited vaccine supply. Using the model, the researchers found that a restricted vaccine supply for 1 million people will not be enough to control the transmission. All scenarios show a large number of cases and deaths after 500 days. Targeting the age group 10–29 for vaccination will reduce the number of cases while targeting people over 65 will affect deaths.
If there are enough doses available to vaccinate the entire NSW population, vaccine efficacy and speed of vaccination will affect the results. At least 75,000 people will need to be vaccinated per day to terminate the transmission rapidly.
Herd immunity can be achieved by vaccinating 66% of the population with a vaccine of 90% efficacy. At a vaccine efficacy of 60%, the entire population will need to be vaccinated to achieve herd immunity. Contact tracing and ring vaccination will require fewer vaccine doses if a more significant proportion of contacts are traced.
Implications of different strategies
Mass vaccination with a high efficacy vaccine represents the best strategy for economic recovery.”
For a population of 7.5 million people, a limited supply of vaccines will not curb virus spread based only on vaccination, and other non-pharmaceutical interventions will be needed.
If the COVID-19 vaccines are effective even after exposure to the virus, or post-exposure prophylaxis (PEP), the best approach will be tracing contacts of confirmed cases and vaccinating them. However, this will require a high capacity for contact tracing. Even though there is no data yet on the efficacy of the COVID-19 vaccines as PEP, the model suggests even with a 50% decrease in efficacy, ring vaccination can be effective, especially with a limited initial supply of vaccines.
With slow mass vaccination, the disease will be around for much longer with its societal and health implications. Rapid mass vaccination will be the most effective in reducing deaths and the number of cases. However, this will require greater planning and the ability to deliver at least 300,000 doses per day. This may be difficult, particularly for a country like Australia, where around 29% of the population lives in remote and rural areas.
In Australia, vaccination is done mainly in primary care. About five million doses are delivered across the country each year under the National Immunization Program. Mass COVID-19 vaccination will require this to increase five times. Adult vaccination is more challenging because of the mobility of adults. Choosing a vaccine with 90% will help, as then vaccinating 66% of the population will achieve herd immunity.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.