What is R0?

R₀, or the basic reproduction number/rate, refers to the contagiousness and transmissibility of infectious pathogens. R₀ varies depending on a variety of factors and is critical in public health management to ensure infectious epidemics (or global pandemics) are controlled.

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R₀ in an estimate of the speed at which a particular infectious disease can currently spread through a given population. Specifically, it refers to the number of people that one person can transmit on average.

Typically, the R₀ varies between <1 if the disease is controlled or not spreading too quickly. If R₀ is 1, then 1 person is capable of spreading to 1 other person on average. If R₀ >1, then the disease can spread to a wider population (exponentially) from one single person, thus potentially creating an epidemic or pandemic.

How is R₀ Calculated?

R₀ is normally calculated based on 3 parameters – 1) duration of contagiousness after infection, 2) the likelihood of infection between the affected individual and susceptible individual & 3) contact rate.

R₀ can further be affected by environmental factors, public health resources, policy/enforcement, the geographical environment, preventative measures, and the presence of immunity (acquired or through vaccination).

Thus, a regional R₀ cannot be applied elsewhere where the disease dynamics, policy measures, and the environment differ, despite the disease ‘infectiousness’ being the same.

If R₀ is less than 1, then the rate of new infections is slowing down across the population and if it remains below 1, then the disease will disappear from that population. If R₀ is equal to 1, then 1 infected person can infect 1 other person thus keeping the infection rate steady and plateaued, but this will not cause an epidemic in that population.

However, if R₀ is greater than 1 where 1 person can infect more than 1 person (e.g. R₀=2, then 1 person infects 2 people, and those 2 people infect 2 people each, thus 4 people, and the rate exponentially increases) leading to an epidemic – and if not controlled, a global pandemic.

As mentioned, many factors influence R₀ and typically in a novel outbreak (like COVID-19, discussed below), the assumption is that no one in the population has prior immunity with no effective vaccines or treatments to control the spread.

R₀ and Historic Infectious Outbreaks

The 1918 influenza (swine flu) pandemic killed 50 million people and had an R0 between 1.4-2.8. As the disease was novel it was far more deadly initially due to lack of immunity, but once it has re-emerged in 2009 (H1N1), the R₀ was below 1.6 due to the combination of vaccines and drugs available.

Common outbreaks of the past:

• Measles – 12-18
• Chickenpox – 10-12
• Polio – 10-12
• HIV/AIDS – 2-5
• SARS – 0.19-1.08
• MERS – 0.3-0.8
• Common Cold – 2-3
• Ebola – 1.56-1.9
• Seasonal Influenza – 0.9-2.1
• 1918 Influenza Pandemic – 1.4-2.8
• 2009 Influenza Pandemic – 1.4.1.6
• COVID19 – 1.94-5.7* (*current estimates vary; see below for more discussion)

Measles, mumps, and chickenpox are the most infectious of all the well-known diseases. Thankfully, through the development of vaccines and medications, these diseases are no longer a global threat, aside from those that refuse to vaccinate their children who are endangering the lives of potentially millions of children worldwide.

There is very little evidence of any danger with vaccination or any risk of autism with the MMR vaccination, and the original study by Andrew Wakefield was fraudulent and false and Wakefield himself disgraced.

R₀ and the COVID-19 Pandemic

The global COVID-19 pandemic (2019/2020) is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which originated from Wuhan in Hubei province in China. Within 3 months after the initial outbreak, the WHO declared the outbreak a pandemic, with many nations enforcing lockdowns to try and combat the outbreak.

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Many different nations were affected differently, and other factors such as the mean age of the population, healthcare system status (critical care beds/ventilators/access), public health strategies, and lockdown measures, all played a significant role in curbing the local epidemics within each nation.

Estimates for the R₀ for COVID-19 vary considerably, but values range between 2.2-2.7, although some estimates place the R₀ at around 5.7. This value was based on the assumption that the virus incubation period was around 4.2 days (time from exposure to symptoms) and a disease doubling time of 2-3 days.

As such, the COVID-19 pandemic is more contagious than the seasonal flu, the 2003 SARS virus, and perhaps even the common cold. The presumed reason why COVID-19 is more contagious than SARS could be due to a much higher affinity of the virus to its receptor.

Furthermore, at this R₀, at least 80% of the population needs to be immune to stop spread or prevent another epidemic. This immunity can come from vaccination (which at present is not available), or through ‘herd immunity’ – something that was originally discussed in the UK before lockdown measures were initiated.

Until a vaccine has successfully passed trials and made in abundance for the population (with those at risk first receiving it), specific measures must remain enforced.

The best way to combat and reduce the R₀ of COVID-19 is through active surveillance (enhanced testing) and declaring COVID-19 history through the use of apps and tracking & tracing of contacts.

Furthermore, quarantine of 14 days of suspected individuals (i.e. those in contact with a confirmed individual, or returning from a travel destination) as well as strict social distancing measures of at least 1.5m (or 2m in the UK), as well as enhanced personal hygiene by washing one's hands with hot water and soap for at least 20 seconds. Shielding the elderly (over 70s) and those with chronic health conditions or compromised immune systems is also essential.

In summary, R₀ is the average rate of spread of an infection in a given population within a particular area – where R₀=1 means that 1 infected person can infect 1 other person – and numbers greater can lead to an epidemic.

Various different parameters and factors influence R₀ such as healthcare systems, public health policies, and lockdown measures in new outbreaks, such as for COVID-19. Successful lockdown measures (including social distancing) and the development of vaccines can reduce R₀ preventing future epidemics.

References

• Ridenhour et al, 2014. Unraveling R0: Considerations for Public Health Applications. Am J Public Health. 104(2): e32–e41. https://pubmed.ncbi.nlm.nih.gov/24328646/
• Delamater et al, 2019. Complexity of the Basic Reproduction Number (R0). Emerg Infect Dis. 25(1): 1–4. https://pubmed.ncbi.nlm.nih.gov/30560777/
• Zhang et al, 2020. Estimation of the reproductive number of novel coronavirus (COVID-19) and the probable outbreak size on the Diamond Princess cruise ship: A data-driven analysis. International Journal of Infectious Diseases. 93:P201-204. https://www.ijidonline.com/article/S1201-9712(20)30091-6/fulltext
• Sanche et al, 2020. High contagiousness and rapid spread of severe acute respiratory syndrome coronavirus 2. Emerg Infect Dis. 2020 Jul (ahead of print) https://doi.org/10.3201/eid2607.200282

Further Reading

• All Coronavirus Disease COVID-19 Content
• What Mutations of SARS-CoV-2 are Causing Concern?
• What is the Clinical Impact of COVID-19 on Cancer Patients?
• Can Pets Get COVID-19?
• An Overview of the SARS-CoV-2 Vaccines

Written by

Osman Shabir

Osman is a Neuroscience PhD Research Student at the University of Sheffield studying the impact of cardiovascular disease and Alzheimer's disease on neurovascular coupling using pre-clinical models and neuroimaging techniques.