Table 1

Factors favouring the eradicability of vaccine-preventable diseases with comparisons between smallpox, polio and COVID-19 (graded for the relative strength of favourability in supporting eradication*)

 Factors favouring eradicability Smallpox (eradicated) Polio (2/3 wild serotypes eradicated) COVID-19 Technical factors favouring eradicability of vaccine-preventable diseases (from Hinman 19994) A highly effective and safe vaccine (preferably cheap and quite stable) +++Combined with ring vaccination of contacts this proved to be a ‘great success’ in smallpox eradication20 +Suboptimal effectiveness where high enteric infection burdens exist; major problems with vaccine-derived poliovirus spread21 +Uncertainty around length of protection, some adverse effects concerns with some vaccines; and cannot be used for contact management. Nevertheless, mRNA vaccines are likely to be further improved and there is potential with intranasal spray vaccines22 Lifelong immunity after natural infection or immunisation +++Immunity probably persists for decades23 ++See row above, although immunity probably life-long after natural infection +Not known but data suggest robust immune response to COVID-19 vaccines in general, and especially mRNA vaccines A short period of communicability (no long-term carrier state) +++ +++ +++ A highly characteristic clinical syndrome (preferably with no or few unapparent infections) +++Typically very distinctive skin lesions Zero scoreOnly around 24% of those infected develop clinical signs which are typically mild and non-specific21 +While around 30% of infections are asymptomatic,24 illness is typically more severe and some symptoms are more specific than for polio, for example, anosmia An easy and reliable means of diagnosis +++See row above +Typically needs a laboratory test for diagnosis; acute flaccid paralysis occurs in 1 per 200 to 1 per 1000 cases +Needs a laboratory test for diagnosis The absence of a non-human (or environmental) reservoir +++No such reservoirs +++No such reservoirs ++There is some risk that other reservoirs might become established—see main text A genetically stable causative agent, and seasonality of occurrence +++Stable and some evidence for seasonality25 ++Stable and seasonal in temperate zones but not the tropics26 +New variants may be a problem with some vaccines27; seasonality still unclear Additional technical factors favouring eradicability (author additions) Relatively low transmissibility (basic reproduction number, R0) resulting in low population immunity threshold (PIT) and greater ease of achieving and sustaining sufficient vaccination coverage ++R0=4.5, PIT=78% (source28) +R0=6.0, PIT=84% (source28) +Initially R0=2.5, PIT=60% (source28), but as of mid-2021 with new variants circulating these figures are probably now much higher in most settings Vaccination can be supported by PHSMs, for example, border controls, physical distancing, hygiene, improved ventilation, mask use, contact tracing (with quarantine and isolation) and community engagement +++Contact tracing with ring vaccination of these contacts proved to be a ‘great success’ with eradication20 +Sanitation improvements can potentially play a role but are far more expensive than vaccination. Community involvement assists with national immunisation days29 +++Important with PHSMs achieving elimination before vaccines were available in various Asia-Pacific countries10 Environmental surveillance can contribute Not scored.Not included in the scoring as it is not relevant due to the highly characteristic clinical syndrome—see above. +Wastewater testing is used and can focus enhanced vaccination efforts30 ++Wastewater testing31 is proving valuable in elimination settings (allowing enhanced additional surveillance) Additional socio-political and economic factors favouring eradicability (author additions and building on Dowdle 19981 and Hinman 19994) Governments can manage disease control messaging (eg, promote vaccination and/or PHSMs) +++Largely true for the eradication period 1959 to 1979 ++Largely true for much of the period 1988 to 2021, although eradication is ongoing for one serotype in two countries +Media, and especially social media, are now less favourable to the goals of government health messaging. Actual governments are involved in ‘antiscience aggression’13 Public and political concern about the health burden from the disease (including on health inequalities) ++At the time the programme started in 1959: probably moderate concern in the 59 countries with endemic disease; total of ≥20 million cases/year32 +At the time the programme started in 1988: probably some concern in the 125 countries with endemic disease; total of 350 000 cases/year32 +++Very high in nearly all countries; an estimated 7.1 million deaths globally from January 2020 to 3 May 202133 Public and political concern about the social and economic impacts of the disease (from illness and control measures required) +As per the row directly above; but also around the ongoing cost of vaccination in both endemic and non-endemic countries +As per the row directly above +++Very high in 2021 in all countries. In the USA alone trillions of dollars have had to be spent because of the pandemic34 Public acceptability of control measures needed to achieve eradication (vaccination and PHSMs) +++High acceptability given the very targeted interventions such as ring vaccination ++Generally high acceptability of the vaccine, although occasional problems in some settings ++Problems with vaccine hesitancy in some countries as of mid-2021 and resistance to some PHSMs, for example, mask wearing Relatively low up-front cost of achieving eradication +++Relatively low cost at US$298 million in 1970s dollars35 ++Polio eradication efforts cost$16.5 billion (US, nominal) for 1988–201836 +Largely unknown, but the costs are probably vast (eg, $66 billion to just vaccinate populations in low-income countries).37 Major upgrading health systems in low-income countries might also be needed Relatively favourable benefit to cost ratio of attempting and then achieving eradication +++Very large benefit to cost ratios, ‘probably the greatest global public investment in human history’35 ++Estimated incremental net benefits of eradication are very large at US$28 billion in 201936 (but there is still uncertainty around ultimate success—that is, the remaining serotype) +Unknown, but the benefit to cost ratio might still be very favourable given the possible high cost on health systems of endemic disease and if repeat vaccinations are required (as seen with influenza vaccination)38 Level of global cooperation to achieve the collective goal of eradication ++Initially the campaign ‘suffered from a lack of funds, personnel, and commitment from countries, and a shortage of vaccine donations’.39 But cooperation subsequently improved +There have been funding shortfalls29 and in 2021 there was a high-income country (the UK) which cut funding support by 95%40 +There have been vaccine shortfalls with the COVAX programme: ‘about 200 million doses behind where we want to be’.41 There are also concerns about ‘new vaccine nationalism’ and government-mediated ‘antiscience aggression’13 Totals and means (with the highest score being +++ for each of the applicable categories) Total=43/48; mean=2.7 Total=26/51; mean=1.5 Total=28/51; mean=1.6
• *Our preliminary assessment for the likely relative strength of favourability for each factor in terms of supporting the eradicability of each pathogen using the following scale: +++ high; ++ medium; + low, and ‘zero score’.

• PHSMs, public health and social measures.