C.3 Examples of future pandemic scenarios

Below we give a narrative description of four stylised scenarios. For readers interested in a more comprehensive overview, Tables 2 and 3 will be of interest – and some readers may want to start with those tables before reading the following narratives.

(1) Good scenario: Low R0 = 2.0, low virulence with IFR = 0.5 percent, highly visible (and therefore amenable to self-isolation, contact tracing and quarantine measures)

This scenario is a bit better than initial SARS-CoV-2 both in terms of R0 (the ancestral variant of SARS-CoV-2 had an R0 of about 2.5) and IFR.

With good preparation and good anticipated vaccines (in terms of both coverage and effectiveness) that can be rolled out in six months, this scenario looks ideally manageable with an elimination strategy (Table 2).

However, the situation might evolve differently. For example, if vaccines were likely to be delayed and there was pressure from citizens to keep borders open, mitigation might offer an alternative route. This would be more feasible and acceptable if those most vulnerable to serious illness (such as people who were older, frail and/or with co-morbidities) could be ‘shielded’ through measures such as distancing and masking, allowing others at low risk of serious illness to become infected and develop something like herd immunity that is later topped up through vaccination.

Another option would be to use elimination initially, hoping for effective vaccines to become available in a short timeframe. But if in the first few months it became clear that vaccines were a way off, or would likely be of low effectiveness, it may make sense to pivot to suppression or mitigation.

All options would be easier to navigate with better preparation. For example:

• Greater built-in ventilation of public buildings would ‘take the edge’ off the R0 (although we do not yet fully understand by how much), probably making it less likely for outbreaks to occur, and – where outbreaks do occur – a bit easier to stamp out.
• A stronger public health workforce with greater capacity to surge contact-tracing and isolation functions would make it easier to control outbreaks without having to resort to more stringent public health and social measures such as lockdowns.
• Greater laboratory capacity and ability to surge testing rapidly would enable faster identification and isolation of cases, and quarantine of contacts.
• A fine-tuned system that can deliver wage subsidies rapidly to regional employers and employees in a targeted manner would assist any need (if at all) for short, sharp regional lockdowns.

(2) Not quite so good scenario: Low R0 = 3.0, low virulence IFR = 0.5 percent, poorly visible (and therefore less amenable to self-isolation, contact tracing and quarantine measures)

Imagine, now, that the pathogen is essentially the same, except that it has some mix of a shorter incubation period, greater infectivity before symptoms, and more asymptomatic infection (Table 2). An elimination or exclusion strategy might still work well, especially if borders are moved rapidly to (good) quarantine systems before any infection takes hold onshore. But if outbreaks occurred, they would be harder to stamp out given less visibility of early infection.

This more challenging set of circumstances might prompt a pivot to mitigation, especially if social licence for an elimination strategy was low, vaccines seemed a long way off, and the vaccines in the pipeline did not appear to have high effectiveness.

As with all the possibilities outlined, better preparation would give decision-makers more options in such a scenario.

(3) Bad scenario: high R0 = 6.0, high virulence with IFR = 7.5 percent, but high visibility

This scenario is in Table 2. Because of the high R0, stamping out or keeping low transmission will be challenging. (An early exclusion response might be ideal, preventing any onshore transmission – or at least holding it off as long as possible until a breach in international quarantine occurs.) But with such a high IFR, there would be a strong incentive to keep transmission low – meaning society would likely be willing to forego liberties to lessen health loss and health system pressure. Better preparation would make an elimination strategy easier. International borders would probably need to be strictly managed with quality quarantine of international arrivals.

Even if vaccines appeared to be a long way off, if an elimination strategy was holding and transmission within the country was very low or at zero, an ongoing elimination approach would probably be better than pivoting to mitigation (which would be likely to bring tens to hundreds of thousands of deaths, and substantial health system and social disruption).

If an elimination strategy failed (from one or a combination of one too many outbreaks, infection taking off, societal fatigue with restrictions, or societal pressure to open up borders), the pivot to a suppression or mitigation strategy would be extremely challenging for health systems with substantial loss of life – unless those most vulnerable could be effectively protected and shielded through measures like masking and distancing.

If an elimination strategy was retained, its true success would likely be a function of vaccines – both uptake and effectiveness. If those factors were poor, health loss would still be substantial over the whole pandemic – better than if the government pursued a suppression or mitigation strategy (rather than an initial elimination strategy) from the outset, but still far from good.

(4) Really bad scenario: high R0 = 6.0, high virulence with FR = 7.5 percent, but low visibility

This scenario is the same as the last, but for a pathogen that has some mix of short incubation period, infectiousness before symptoms and moderate to high asymptomatic infection (but still infectious). Contact tracing and citizen self-isolation upon becoming symptomatic are unlikely to be very effective in this scenario. Unless borders were shut before infection arrived (that is, an immediate exclusion strategy), stamping out incursions and outbreaks would be difficult – requiring luck, an extremely good public health workforce with strong surveillance and contact-tracing systems and likely repeated stringent population-wide PHSMs to help stamp out outbreaks.

Under this scenario, the likelihood of losing control and infection taking off requiring a mitigation strategy is high – but that mitigation strategy would still require stringent PHSMs to protect health services during waves of infection. It would be very challenging if elimination failed, with substantial health, societal and economic loss.

An early and effective vaccine would be desperately sought; countries that had invested in vaccine production and access schemes in advance would be advantaged.

Table 2: Pandemic scenarios for low or moderate infectiousness (R0 = 2.0 or 3.0) and low virulence (IFR = 0.5 percent) pathogen

^† Assumed no international border quarantine in loose suppression or mitigation.

^‡ Assumes strong immunity from natural infection that neither wanes nor is ‘broken through’ by new variants.

Table 3: Pandemic scenarios for high infectiousness (R0 = 6.0) high virulence (IFR = 7.5 percent) pathogen

^† Assumed no international border quarantine in loose suppression or mitigation