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 is 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.
The reproduction rate 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).
R0 itself only applies to a population when everyone within it is vulnerable. This means no one has already had the disease, there is no control of its spread, and no vaccination has been carried out.
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 it is typically applicable in the beginnings of a novel outbreak, where there is an assumption that no one in the population has prior immunity and there are no effective vaccines or treatments to control the spread.
Once part of a population becomes immunized or control measures such as social distancing are put in place, the effective reproduction number (Re), the number of people who can be affected by an individual at any specific time, becomes the appropriate measure. R0 and Re are often confused, and may be generally referred to as the R number. It is important to recognize that the reproduction rate will differ where the disease dynamics, policy measures, and the environment differ, despite the disease ‘infectiousness’ being the same.
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
Some infectious outbreaks of the past and their estimated median r0 numbers are:
- 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 – 0.4-5.7* (*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 instances where vaccination is refused.
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 in Wuhan, China. Within 3 months of 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 but values range between 0.4-5.7. R0 values often vary depending on the data and models used to calculate it.
As such, COVID-19 is likely to be more contagious than the seasonal flu, the 2003 SARS virus, and perhaps even the common cold. The reason for COVID-19 being more contagious than SARS is thought to 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 from COVID-19 to stop spread or prevent another epidemic. This immunity can usually come from vaccination or through ‘herd immunity’.
Herd immunity is achieved when enough of a population have had a disease and gained immunity from it to stop the spread. However, reinfection is now known to be possible in COVID-19. Recent research suggests that those infected with the virus are protected with immunity for five months. Hence, vaccination is a more appropriate strategy in the case of COVID-19.
It is also important to note that many variants of COVID-19 have arisen since the beginning of the pandemic. As recent variants have been found to be more infectious, they are have affected the reproduction rate.
The best way to combat and reduce the Re 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 6 feet (around 2m), and enhanced personal hygiene by washing one's hands with hot water and soap for at least 20 seconds, should be employed. Shielding the elderly (over 70s) and those with chronic health conditions or compromised immune systems is also essential.
References
Further Reading
Assessing COVID-19 vaccine effectiveness against long COVID