The novel coronavirus (SARS-CoV-2) has already made a significant impact on the world, with its rampant spread and persistent nature. This article seeks to understand the intricacies of the virus’s spread and the necessary preventive measures to combat it.
The Basic Reproductive Ratio (R0)
The Basic Reproductive Ratio, pronounced as R-nought (R0), gives an average of how many persons a contagious individual is likely to infect. With a high R0, the virus becomes highly contagious and fast-spreading. The value of R0 depends on three aspects: the duration of infectiousness, the number of vulnerable individuals, and the probability of susceptible people getting infected.
Social distancing is a crucial tool that can reduce R0 effectively. However, distancing only from symptomatic individuals may not suffice as asymptomatic individuals can also transmit the virus. Therefore, the spread of Covid-19 and human behavior significantly affect R0.
Understanding the Functioning of R-nought
An individual infected with SARS-CoV-2 can remain infectious for up to two weeks. In the initial phases of the virus’s spread, there are numerous susceptible people to infect. The R-nought of the SARS-CoV-2 virus is estimated to be between 2 and 3 which signals its contagiousness. For instance, if we assume an R0 of 2 and an infective period of 10 days, one person would infect approximately 2,046 individuals within ten days. As more people develop immunity, the R-nought decreases, thereby reducing new infection cases.
The Concept of Herd Immunity
Herd immunity refers to a situation when a large proportion of the community becomes immune to an infectious disease, thus preventing further spread. This immunity can be achieved either through infection or vaccination. An infection triggers the immune system to recognize and remember the virus, ensuring protection against future infections.
The lack of a vaccine for SARS-CoV-2 makes herd immunity through infection the only current option. However, achieving such immunity comes at the cost of a significant portion of the population getting infected first. In India, for example, vaccines were instrumental in eradicating polio.
Series Interval
The Series Interval refers to the time between two successive symptomatic infections during an outbreak. This interval sheds light on the spreadability of the virus; a shorter interval corresponds to a faster spread. With SARS-CoV-2, the Series Interval is estimated between 5 and 7 days compared to 1.3 days for normal influenza. This indicates a slower community spread of Covid-19 and, consequently, a slower build-up of herd immunity, potentially extending the pandemic duration.
Herd Community Threshold and Lockdown
The herd immunity threshold defines the proportion of immunity among individuals to prevent disease spread. This value can be calculated using the formula 1-(1/R0). The lockdown can be safely lifted once about two-thirds of the population achieves immunity. However, getting a precise estimate of the number of immune individuals is challenging and it requires surveillance testing in communities.
Preventive Measures
Contact tracing, isolating the infected individuals, and quarantine measures play a crucial role in reducing R0. Identifying the most affected geographical regions and implementing stricter lockdowns, intensifying surveillance-testing in these areas, and isolating the infected individuals are necessary steps.
It is essential to ban large gatherings, including religious and political events. Government support in the form of food and basic amenities for those affected by the lockdown is needed. Loosening lockdown restrictions for daily wage earners should also be considered, but with strict maintenance of social distancing and expanded symptom-monitoring and community-testing. Regular surveillance testing and monitoring should continue at all levels to identify new pockets of high-intensity infection.