Day Zero — the precise date that the virus broke into the human species, as a “zoonotic” virus from an animal host to a human host. As an addendum, Patient Zero is yet to be definitively identified. Day Zero has at various times been thought to be 31 December, 1 December, 17 November or perhaps as early as October. The starting point along with the dynamics — discussed below are crucial inputs into designing the most appropriate health policy response.
R-0 or R-Nought, the basic reproduction rate — how many people can an infectious host infect (where “nought” refers to the absence of individual or herd immunity, which is assumed to be the case when a new virus is introduced into a population on Day Zero). Effective reproductive rates are estimated from observed infections and contacts (assuming that testing and contact tracing are representative and accurate).
Case or infection fatality rates (CFR or IFR — deaths divided by infections). In the case of COVID-19, as is often the case in the early stages of novel virus outbreaks, especially where R-0, R-Eff, or Day Zero / Patient Zero have yet to be determined, it can be quite hard to determine how deadly the virus actually is.
Asymptomatic “presentation” — when individuals test positive, do not exhibit known symptoms but are (probably) infectious: Is it that we still do not know all the symptoms, and hence don’t know everything to look for in a victim? Or that symptoms arise later in some cases? Are asymptomatic carriers immune?
Estimates of R-0, R-Eff and IFRs have varied widely over time, and across countries and even regions within countries. If R > 1, infections would rise exponentially. Indeed, it is impossible to be sure to what extent increases in infection numbers and rates and their subsequent decline is due to changing testing numbers or patterns (e.g., whether tests are random or there is deliberate “sample selection bias” to verify or falsify infection, or test front-line health workers).
If R < 0, then infections would fall exponentially over time, tending towards viral extinction. Of course, if lockdowns and social distancing are the main reason for declining infections – i.e., in the absence of herd immunity, a cure or a vaccine – then release would probably lead to a resurgence in infections. Such developments would be seen in infectious but asymptomatic cases contributing to earlier compartments in Figures 4 and 5 above. If immunity is not conferred via infection and recovery and instead, re-infection and illness / death can occur, R-Eff and IFR would both trend higher and would call for renewed social distancing or lockdowns, in the absence of effective vaccination or treatment.
^The Curious Case of COVID-19: Broad Public Health Policy Dilemmas
Judgments about such issues and variables are crucial to help determine the best course of action: Whether to rely on “Non-Pharmaceutical Interventions” — such as lockdowns, social distancing and self-isolation to slow the spread of the disease lest public health systems are overwhelmed perhaps more than once, if there are substantial second waves? Do flatter curves reflect the temporary effect of the lockdowns themselves, such that the virus is likely to make a significant comeback following even partial relaxations?
Or if the virus had already propagated for months or a couple of quarters instead of some weeks in a highly interconnected world, before the lockdowns began, might it be better to shift gears, and instead to emphasize mass testing and contact tracing, targeting restrictions on high-risk categories; and mass
“serological” testing for antibodies to establish both infection and possible immunity levels across populations? Does this amount to a judgment that herd immunity is within reach?
“Herd Immunity” vs. “Non-Pharmaceutical Intervention”: An open question. Are we better off as a species, as societies, as individuals – whether from public health or economic perspectives — by allowing the virus to proceed to encourage the build-up of immunity, or by trying to maintain or restore NPIs? We don’t know enough about the virus itself or its long-term impact (e.g., there may be substantial permanent lung damage for victims, even after general recovery, as is believed to have occurred with prior coronaviruses that cause respiratory difficulties).
Under these circumstances, it is probably best to be “conservative” to maintain various forms of social distancing to at least reduce risk (including isolation of the known vulnerable and minimising risk of exposure in confined, crowded spaces), personal protective equipment (e.g., masks) and be prepared for partial lockdowns (i.e., “adaptive release”) should secondary waves emerge, as they have in some of Asia’s “First-In, First-Out” countries.
“Endemic” vs. “Epidemic” vs. Viral Extinction: COVID-19 is probably here to stay. Can we eliminate the virus (extinction) or do we need to figure out how to live with it? — as by developing a vaccine (or even a cure) or acquiring immunity (perhaps through repeat waves or epidemic/pandemic episodes) — because it’s out in the world and cannot be eliminated (that is, it will become “endemic”, like seasonal flu)?
Like all else to do with COVID-19, it’s hard to be sure, but the “conservative” approach, again, would likely be to expect COVID-19 to become endemic, not least because the approach in different countries has been very different. The chances of viral extinction (i.e., when R-Eff < 1 in sustained fashion) may have been achieved in easily isolated environments like New Zealand or Iceland. The only major country with data pointing to viral extinction is China, but even there, cases imported from Russia are an issue; plus the emergence of more cases including asymptomatic and domestic transmission and more deaths than originally recorded points to uncertainty. Lockdown relaxations, especially if premature when curves have yet to sustainably flatten, would probably contributed to sustaining the virus within the human population.
^“The Cure should not be Worse than the Disease” — Assessing which requires us to know enough
Figure 6 attempts to summarize the epidemiological, medical and scientific evidence. COVID-19 seems to be significantly more lethal and infectious than the seasonal flu and is probably more infectious though less lethal than the Great Influenza Pandemic of 1918.
Though there have been several important pandemics since, the so-called “Spanish Flu” a century ago is perhaps in many respects the most important and relevant precedent. It came in multiple waves, spreading around the world, infecting perhaps 500 million – then about a third of the global population. The economic consequences of that pandemic appear to have included a contribution to the subsequent US depression of the early 1920s, which were followed by the Roaring ’20s.
Though we are heading for a tentative lockdown relaxation on the basis that the worst is past, the battles to contain COVID-19 are far from over – given the prospect of periodic waves as a century ago. The remaining uncertainties suggest that the releases will remain partial and gradual — sector by sector, region by region, country by country; and very different from country to country in terms of timing and extent.