Throughout the Covid-19 pandemic we have been repeatedly bombarded with a statistic called R0 (often referred to as the “basic reproductive rate”), whether from the Government or the news channels, as a way of describing how SARS CoV2 is spreading in the community. By now most of know what it is; it’s the number of people that a person with an infection will infect. If R0 >1 then the initial “outbreak” is expanding, if R0 <1 it is shrinking.
But, the Government and the news have been using it incorrectly… and as a result so have a lot of us. Let me explain….
Okay, R0 IS the number of people a person with an infection will spread the infection to, that bit is correct.
The first thing that is wrong with how R0 is being portrayed is the name is wrong. R0 is not a rate, it is a number. The correct name is the “basic reproductive number” not the basic reproductive rate. A rate is a “speed” at which something changes, it is per second, or per hour, etc. it is related to time. R0 is just a number, the number of people infected by 1 person. So, it is the “basic reproductive number”.
Another thing that is wrong is that R0 DOES NOT change over time. R0 is the “basic” reproductive number… it’s the number at baseline BEFORE any interventions occur, i.e. at the time the infection first enters a community. When SARS CoV2 first entered the UK R0 was 3. It assumes no one has been vaccinated, no one has had the disease before and there are no control measure available… the infection can spread as much and as easily as it likes! It’s the unchecked virus left to do what it would naturally do e.g. worst case scenario.
Why does this matter? Well, perhaps it is a bit semantic, but if you use the wrong name then people will get confused and make mistakes. For example, if someone looks up R0 for Covid-19 hoping to see what the current R value is then they will be confused because R0 hasn’t changed since the beginning of the pandemic, it is still about 3.
So, what should we be using to track how the pandemic is going? The correct value is called Rt or the “effective reproductive number” and represents a value of reproduction number as close to real-time as possible i.e. t = time.
How do you calculate either the “R0 or Rt” reproductive number?
Don’t panic, I’m not going to get all “mathsy” on you and start using long equations full of Greek letters and weird symbols… I can’t get my head around that either. Instead let’s look at what affects the reproductive number (R0 or Rt) in more general terms.
The probability of getting the infection if you come into contact with the infection (r) is dependent on lots of factors such as the mode of transmission, how much of the organism is needed to set up the infection, how long the exposure was, etc. Airborne organisms (e.g. SARS CoV2) tend to have higher probabilities of infection as they are easy to cough or sneeze on each other, but wearing a face mask should reduce this probability. Infections which need only a few organisms to spread (e.g. Norovirus) need strict infection control measures, whereas infections that can spread with only brief contact (e.g. Ebola Virus) need to have excellent quarantine procedures in place! The concern about variants of SARS CoV2 is that they will spread more easily (i.e. be more infectious) and therefore have a higher probability of infection.
The rate of contact with the infection (c) is the number of times an infected person comes into contact with susceptible people (uninfected and non-immune people). It is a measure of the number of opportunities the infection has to spread. If an infection is asymptomatic in lots of people (approximately 30% of Covid-19 infections) or causes few symptoms, such that the infected person goes about their normally daily activity and meets lots of other people, the rate of contact will be high and this exacerbates the contact ultimately multiplying the probability. However, the more people who are immune to the infection, either through vaccination or natural immunity from a previous infection, the lower the rate of contact with susceptible people. Quarantine of infected people, whatever their symptoms, and vaccination are good the ways to control the rate of contact.
The duration of infectiousness (d) is the time period in which someone with an infection can pass that infection onto another person. For SARS CoV2 this is from approximately 2 days before until 10 days after onset of symptoms i.e. 12 days. The longer the period of infectiousness the greater the chance that someone will pass the infection on, this again exacerbates the spread ultimately multiplying the probability. Sometimes medications can reduce the period of infectiousness (e.g. antibiotics in bacterial infections) but for most viruses there are no effective ways to reduce the duration of infectiousness. This is where test and trace should help, and quarantine is needed. Test and trace should identify those who have been exposed and who may become infectious to others (even without symptoms), and if they then quarantine during the time they are infectious (14 days after exposure) then transmission is interrupted. This is why strict “you’ve been exposed – please isolate” messages are essential, even if not abided by; “I feel OK” is not a reason to not self-isolate!
One more important thing!
The last important thing to remember about Rt is that it is not a fixed figure. It is always changing. It changes over time and it is different in different places. We get told that numbers of infections are rising in the UK, but they are not rising at the same “rate” everywhere, some places are worse than others. Places with high rates of vaccination and good compliance with infection control procedures have low reproductive numbers and low numbers of infections; places with low vaccination rates and poor compliance with infection control procedures will have high numbers of infection and high reproductive numbers.
This is why the UK Government initially tried to use Tiers of lockdown measures based on what was happening in specific places to control the pandemic. It’s a good way to tackle localised problems. So why has this method stopped… because people complained it was unfair!!!
People were concerned areas were being picked on if they had high numbers of infections and high reproductive numbers and implied they were not following guidelines, self-isolation, getting a vaccine etc. It wasn’t unfair, it was sensible and in my opinion it is actually unfair to have a national “one size fits all” policy which is applied to areas with very low reproductive numbers… but I’m not a politician who has to be popular and win votes to keep my job 😊. In fact pandemics seem to make a Microbiologist’s job security better!
Hello Boris, have a read of this and think about whether you need to go back to a Tier system in July… it won’t be popular but it will be more effective…
R0 is static and stays with the virus, the news and politicians are using Rt and this changes all the time, place to place… but how do we re-educate ourselves to use the correct terminology now we are all experts on R0!? Interestingly, the Government are now calling it the “R Value” so maybe the education is working… the problem is the Rt is currently between 1.2 and 1.4 so the pandemic is getting worse again in the UK… whoops! (Remember if Rt >1 then the ongoing “outbreak” is expanding, if Rt <1 it is shrinking).