I have mentioned the Basic Reproductive Ratio (R0 or R Naught) in prior blogs. This is the expected number of cases directly generated by one case in a population where no other individuals are infected or immunized. The effective reproductive number (R) is the number of cases generated in the current state of a population inclusive of those who are already infected or who have been vaccinated against the disease. By definition, R0 cannot be modified through vaccination campaigns. It’s also important to note that R0 is a dimensionless number and not a rate, which would have units of time like “doubling time.” That means that even if a virus has a high R0, the speed with which it spreads can vary depending on the average time period it takes for an individual to become infectious and remain infectious.
When I use R0 in this blog, I may actually be using R (effective reproductive number) or rate. All of the data cites R0, so I will use this term as a proxy.
In commonly used infection models, when R0 > 1 the infection will be able to start spreading in a population, but not if R0 < 1. Generally, the larger the value of R0, the harder it is to control the epidemic. So if, on average, one sick person gets one other person sick, the number of active cases will stay the same. If it’s less than 1, the number of infected people will diminish over time. If over 1, the number of infected people will increase over time.
The R0 rate of the flu is in the 1 range with a number around 1.3 in flu season.
The R0 rate of COVID-19 is in the 2-3 range with 2.3 in a recent article I read. I will use 2.3 for future calculations in this blog.
That number does not sound too high. Yet watch what happens as it spreads:
Flu: First person infects 1.3 people. Those 1.3 people infect 1.3 people each, so now 1.69 people have it. Then 2.2. Then 2.8. Then 3.7. Then 4.8. By the 10th transfer, it is a little under 14 people.
COVID-19: First person infects 2.3 people. Those 2.3 people infect 2.3 people each, so now 5.3 people have it. Then 12.2. Then 28. Then 64.4. Then 148. By the 10th transfer – it is around 4,143 people.
It appears COVID-19 is spreading fast because there is limited natural immunity and it has an efficient transfer process (contagious without showing symptoms, survival rate on surfaces, airborne possibilities, etc.). The higher R0 has an extreme impact when compounded over time. This is the main issue with COVID-19 right now. If the basic flu strain had the same R0, it would overwhelm hospital systems too. High RO, combined with panic (every person with any virus symptoms going to the hospital), is what’s causing a medical resource shortage.
What causes the R0 (or R) rate to drop?
Viruses tend to flourish in certain environments and COVID-19, like the flu, appears to dislike heat and sun. Both can kill off viruses on surfaces faster, thus slowing the spread rate. If you look at COVID-19 outbreaks, the large clusters are heavily in the northern hemisphere and north of the equator by some distance. If COVID-19 continues to follow seasonal flu traits, the outbreak clusters will move to the southern hemisphere as weather warms in the north and winter sets in, in the southern hemisphere.
The main thing that slows R0 is creating population or herd immunity. For example, as half of the population gains immunity, in theory, the R0 rate will drop in half (to 1.15, in my example) or very close to the seasonal flu rate. There are really only a few ways to develop immunity. Catch the virus and recover or get a vaccine are the most common. Two others involve receiving the antibodies directly as injections, either lab created antibodies or antibodies from recovered people’s blood. As people catch the virus and develop immunity, they are much less likely to get the virus again (unless it morphs so much the body does not recognize it). Once herd immunity levels are reached, the virus stops spreading at the same rate (fewer hosts, less production factories). Basically, people need to get the virus.
Right now, the immediate goal is to lengthen the time frame of infection so medical systems can ramp up to handle the volume. We cannot have all the immunocompromised people catching the virus at once. As more people get it, the R0 rate will naturally slow anyway. Germany estimated 70% of their population will get the virus. I have seen estimates of 50% in the United States. Understandably, nobody wants a virus, especially one that can sicken some people as severely as this one. Although most people who have it are asymptomatic or experience only mild symptoms, this can hit some people really hard. Any virus can. I vividly recall one time I thought I was going to die from a bug soon after I founded Rising. I couldn’t even get out of bed to get a drink or make a call. For some reason, that virus hit me especially hard. I know a few people that have had COVID-19 and the stories range from no big deal to ICU for one person my age (he had a great reaction to the malaria drug and is doing better).
It’s important to remember that as more people are tested, the number of people that test positive has to grow (especially the total number of people that have had it). Candidly, I don’t think numbers like “newly tested positive” have much value now, as not many people were originally tested so the baseline is inaccurate. It will take a while for the testing numbers to stabilize and become rational, but they will over time. Doubling rate (the time it takes to double the amount of new cases), the total number of people who have recovered, recovered rate (% of total cases), current active cases, death rate (divided by estimated cases or positive tests) are all numbers to monitor to look for trends.
There appears to be some good news on the R rate due to recent precautions. Fox News medical correspondent Dr. Nicole Saphier said there are “very encouraging numbers” showing the “slowing” of COVID-19 cases in New York. Saphier explained that when the “stay-at-home” order was implemented last Sunday, the doubling rate was every two days. “By Monday, it was taking 3.4 days for doubling of cases and then on Tuesday night, they said it was actually 4.7 days for doubling of cases…Even though we’re doing more tests than before in New York, the time that it takes to double the cases is actually elongating” said Dr. Saphier. This would indicate social distancing is succeeding and slowing the spread of the virus.
Stay safe, stay strong!