Dr. Muhammad Torequl Islam :
1. Time for a single infected person to yield one million infected people
The basic reproduction number, R0, suggests each infection directly generates 2-7 more infections in the absence of countermeasures like physical distancing. Once a person is infected, it takes a period of time (latent period) before they are able to transmit the virus. The current best-estimate of the median latent time is ˜3 to ˜4 days of close to maximal infectiousness. The exact durations vary among people, and some are infectious for much longer. Using R0˜4, the number of cases will quadruple every ˜7 days or double every ˜3 days. 1000-fold growth (going from one case to 103) requires 10 doublings since 210 ˜ 103; 3 days × 10 doublings = 30 days, or about one month. So we expect ˜1000x growth in one month, a million-fold (106) in two months, and a billion fold (109) in three months. Even though this calculation is highly simplified, ignoring the effects of ‘super-spreaders’, herd-immunity and incomplete testing, it emphasizes the fact that viruses can spread at a bewildering pace when no countermeasures are taken.
2. How does physical distancing affect?
Suppose that you are infected and you encounter 50 people over the course of a day of working, commuting, socializing and running errands. To make the numbers round, let’s further suppose that you have a 2% chance of transmitting the virus in each of these encounters, so that you are likely to infect one new person each day. If you are infectious for 4 days, then you will infect four others on average.
3. Initial quarantine period two weeks, why?
The time required for infection to symptoms is termed the incubation period. The average incubation period for the severe acute respiratory distress syndrome coronavirus 2 (SARS-CoV-2) is approximately 5 days. But, it may vary from person-to-person. Approximately 99% of those showing symptoms will show them before day 14, which explains the two week confinement period. However, this analysis neglects infected people who never show symptoms. The asymptomatic people are not usually tested, therefore, it is yet to be found out how many such infected people remain infectious for.
4. How N95 masks block the SARS-CoV-2?
These are designed to remove more than 95% of all particles that are at least 0.3 microns (µm) in diameter. The particle filtration efficiency of N95 masks shows that they are capable of filtering ˜99.8% of particles with a diameter of ˜0.1 µm. SARS-CoV-2 is an enveloped virus ˜0.1 µm in diameter, so N95 masks are capable of filtering most free virions, but they do more than that. SARS-CoV-2 is often transmitted through respiratory droplets (coughing and sneezing), which are usually divided into two size bins (i) large droplets: >5 µm in diameter (fall rapidly to the ground and are thus transmitted only over short distances) and (ii) small droplets: =5 µm in diameter that can evaporate into ‘droplet nuclei’, remain suspended in air for significant periods of time and could be inhaled.
5. Mutation rate of the virus
The researchers commonly use two measures describing the rate of genomic change (i) evolutionary rate (the average number of substitutions that become fixed per year in strains of the virus, given in units of mutations per site per year) and (ii) mutation rate (the number of substitutions per site per replication cycle). Let’s consider a single site at the end of a year. The mutation rate in a ß-coronavirus suggests that this site will accumulate ~10-6 mutations in each round of replication. Each replication cycle takes ~10 hr, and so there are 103 cycles/year. Multiplying the mutation rate by the number of replications, assuming neutrality and neglecting the effects of evolutionary selection, we will find 10-3 mutations per site per year, consistent with the evolutionary rate inferred from sequenced coronavirus genomes. Therefore, this virus undergoes near-continuous replication in the wild, constantly generating new mutations that accumulate over the course of the year.
1. Time for a single infected person to yield one million infected people
The basic reproduction number, R0, suggests each infection directly generates 2-7 more infections in the absence of countermeasures like physical distancing. Once a person is infected, it takes a period of time (latent period) before they are able to transmit the virus. The current best-estimate of the median latent time is ˜3 to ˜4 days of close to maximal infectiousness. The exact durations vary among people, and some are infectious for much longer. Using R0˜4, the number of cases will quadruple every ˜7 days or double every ˜3 days. 1000-fold growth (going from one case to 103) requires 10 doublings since 210 ˜ 103; 3 days × 10 doublings = 30 days, or about one month. So we expect ˜1000x growth in one month, a million-fold (106) in two months, and a billion fold (109) in three months. Even though this calculation is highly simplified, ignoring the effects of ‘super-spreaders’, herd-immunity and incomplete testing, it emphasizes the fact that viruses can spread at a bewildering pace when no countermeasures are taken.
2. How does physical distancing affect?
Suppose that you are infected and you encounter 50 people over the course of a day of working, commuting, socializing and running errands. To make the numbers round, let’s further suppose that you have a 2% chance of transmitting the virus in each of these encounters, so that you are likely to infect one new person each day. If you are infectious for 4 days, then you will infect four others on average.
3. Initial quarantine period two weeks, why?
The time required for infection to symptoms is termed the incubation period. The average incubation period for the severe acute respiratory distress syndrome coronavirus 2 (SARS-CoV-2) is approximately 5 days. But, it may vary from person-to-person. Approximately 99% of those showing symptoms will show them before day 14, which explains the two week confinement period. However, this analysis neglects infected people who never show symptoms. The asymptomatic people are not usually tested, therefore, it is yet to be found out how many such infected people remain infectious for.
4. How N95 masks block the SARS-CoV-2?
These are designed to remove more than 95% of all particles that are at least 0.3 microns (µm) in diameter. The particle filtration efficiency of N95 masks shows that they are capable of filtering ˜99.8% of particles with a diameter of ˜0.1 µm. SARS-CoV-2 is an enveloped virus ˜0.1 µm in diameter, so N95 masks are capable of filtering most free virions, but they do more than that. SARS-CoV-2 is often transmitted through respiratory droplets (coughing and sneezing), which are usually divided into two size bins (i) large droplets: >5 µm in diameter (fall rapidly to the ground and are thus transmitted only over short distances) and (ii) small droplets: =5 µm in diameter that can evaporate into ‘droplet nuclei’, remain suspended in air for significant periods of time and could be inhaled.
5. Mutation rate of the virus
The researchers commonly use two measures describing the rate of genomic change (i) evolutionary rate (the average number of substitutions that become fixed per year in strains of the virus, given in units of mutations per site per year) and (ii) mutation rate (the number of substitutions per site per replication cycle). Let’s consider a single site at the end of a year. The mutation rate in a ß-coronavirus suggests that this site will accumulate ~10-6 mutations in each round of replication. Each replication cycle takes ~10 hr, and so there are 103 cycles/year. Multiplying the mutation rate by the number of replications, assuming neutrality and neglecting the effects of evolutionary selection, we will find 10-3 mutations per site per year, consistent with the evolutionary rate inferred from sequenced coronavirus genomes. Therefore, this virus undergoes near-continuous replication in the wild, constantly generating new mutations that accumulate over the course of the year.
(Dr. Muhammad Torequl Islam is Assistant Professor, Department of Pharmacy, Life Science Faculty, Bangabandhu Sheikh Mujibur Rahman Science and Technology University. E-mail: [email protected])
