A new modeling study of the United States suggests that vaccination could markedly reduce the rate of infection, hospitalizations and mortality in the current COVID-19 pandemic – even if confined to adults alone. However, the interim period will still require non-pharmaceutical interventions (NPIs) – social distancing, mask-wearing and regular handwashing – to mitigate viral spread until a vaccine is made available and can be distributed to all who are eligible.
Vaccine efforts near the finishing post
The pandemic has speeded up the vaccine development efforts worldwide, as researchers and governments alike strive to put an end to this scourge. This is chiefly due to the absence of any effective clinical or pharmaceutical intervention that can cut short the illness. This has necessitated NPIs like social distancing, masking, and lockdowns in many parts of the world.
Now, after herculean and incredibly fast-paced research, several vaccines are almost at the finishing post, with 12 candidate vaccines in phase III trials. In fact, early efficacy results have been reported by some teams, though with some blips. There are another 80-plus candidate vaccines at various earlier stages of development. The new focus of research is on the expected impact of universal vaccination on the parameters of infection.
According to the profile for an ideal vaccination scenario, as developed by the World Health Organization (WHO), called the target product profile (TPP), the vaccine should have at least 50% efficacy, but the preferred efficacy is 70%. The US Food and Drug Administration (FDA) has set a lower target, of 30% to 50%. The two vaccines from Pfizer and Moderna have reported a 90% or greater efficacy as preliminary findings from their phase III trial.
Nonetheless, vaccine efficacy is not directly correlated with the impact of vaccination on the population, since this depends on a mix of logistical, epidemiological and cost factors.
Modeling vaccine efficacy in the US
To try and understand what this means, the researchers developed a mathematical model built on the agent-based model approach to COVID-19 transmission, setting it to parameters that conform to the characteristics of the US population, and the expected outcomes of infection depending on the age group.
The vaccine was prioritized for healthcare workers and high-risk individuals (including those 50 years old and above, or with comorbid conditions like hypertension or diabetes), first of all. Individuals with such health conditions have a 2-4-fold risk of adverse outcomes. Similarly, advancing age is linked with a steep rise in disease severity and death.
Children under 18 were not considered eligible. The remaining population was allocated vaccine priority in accordance with the guidelines for seasonal flu vaccine distribution, until about 40% of the population was covered. This would include about 40% of those aged 18-19 years, almost half of those between 20 and 64 years, and 70% of those 65 or more.
The efficacy was considered to be 90%, but adjusted for the presence of comorbid conditions from knowledge gained by experience with the flu vaccine. They implemented it as a prime-boost regimen, the two doses being given at a 28-day interval, with a pre-existing population immunity of 10%. They assumed that vaccination would reduce the severity of illness even if infection occurred post-vaccination, and assumed the immunity would last beyond one year. The effective reproduction number was pegged at 1.5 to adjust for the measures currently in force to reduce viral spread.
Prioritized vaccination sharply reduces pandemic impact
The researchers found that the effect of vaccination was to reduce the overall infection rate from around 7% to 1.6%. The largest impact was found in the age group 65 years and above, with the infection rates dropping by 83% to 90%. Among children, the attack rate was predicted to drop by half because of indirect protection. The number of infections averted was predicted to be larger with lower pre-existing immunity.
In the vaccinated population, the rate of hospitalizations-cum-admission to the intensive care unit (ICU), and deaths, were markedly reduced by around 85% and 88%, respectively. Even if only 20% of the population was vaccinated, the reduction in hospitalization and deaths was predicted to be around 60% and 64%, respectively.
What are the implications?
This indicates the significant impact of an effective vaccine, with 90% efficacy, even at low coverage on disease outcomes and transmission. The necessary conditions for this to happen include a high coverage of healthcare workers (57%) and individuals with underlying illnesses (57%), and the execution of strategic communication campaigns publicizing the safety of the vaccine and the protection it offers against severe COVID-19 and death, as well as a near-perfect completion of the two-dose protocol.
Nonetheless, the time required to roll out the vaccine at the national level, even in a developed society like the USA, would be several months. This highlights the need to continue other policies, especially the NPIs, aimed at containing COVID-19, which will cover the gaps left by vaccine hesitancy and unwillingness, as well as vaccine dropouts.
The researchers conclude, “If the arrival of vaccination programs is accompanied by widespread relaxation of other measures, much higher coverage will be necessary with a significantly higher distribution capacity. Nevertheless, our results are an encouraging signal of the power and promise of vaccines against COVID-19.”
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.