Preliminary data from a study reported in the journal BMJ Global Health suggests that the likelihood of global eradication of Covid-19 is more than that for polio, but much less than it was for smallpox. The eradication of Covid-19 may be aided by public health measures in place, vaccination and the joint global efforts and the will towards this common goal.

Public health experts from the University of Otago Wellington in New Zealand compared Covid-19 with smallpox and polio. There was a vaccine for smallpox, which perhaps was the first ever vaccine available and polio vaccines are also available (OPV and IPV).

Researchers identified 17 variables related to technical, economic and sociopolitical factors that have a bearing on the achievement of eradication of vaccine-preventable diseases (smallpox, polio and Covid-19 in this case). A three-point scoring system was used for each of 17 variables.

The technical factors included “availability of a highly effective and safe vaccine, lifelong immunity after natural infection/vaccination, short period of communicability, a highly characteristic clinical syndrome (preferably with no or few unapparent infections), easy and reliable means of diagnosis, absence of a non-human (or environmental) reservoir and a genetically stable causative agent, seasonality of occurrence, relatively low transmissibility, public health measures and environmental surveillance.

The sociopolitical and economic factors included government management of infection control messaging (vaccine, public health and social measures), Public and political concern about the health burden and the socioeconomic impacts of the disease, Public acceptability of control measures needed to achieve eradication, relatively low up-front cost of achieving eradication, relatively favourable benefit to cost ratio of attempting and then achieving eradication and level of global cooperation to achieve the collective goal of eradication”.

The average score for smallpox was 2.7, while for Covid-19 it was 1.6 and 1.5 for polio.

Smallpox scored highly on vaccine, where availability of a safe and effective vaccine combined with the strategy of ring vaccination of contacts successfully eradicated the disease. While, both polio and Covid-19 scored low on vaccine. Vaccine-derived poliovirus continues to be a problem alongwith “suboptimal effectiveness where high enteric infection burdens exist”. There have been reports about the waning immunity after six months post-vaccine. There is also a concern about adverse effects with the vaccine, although the balance tilts in favor of benefits and everyone eligible must take the vaccine.

Immunity after natural infection or immunisation persists for smallpox and also for polio after natural infection, while the duration of protection offered by Covid-19 vaccine is variable.

Smallpox is easy to diagnose because of the typical skin lesions; Covid-19 is mostly asymptomatic, but there are certain symptoms that are highly specific for Covid-19 such as new onset of loss of taste and smell. Polio scored zero on this; “only around 24% of those infected develop clinical signs which are typically mild and non-specific”.

Contact tracing and public health measures proved to be effective for both smallpox and Covid, but this is not the case for polio. According to the authors, improvement in sanitation may have a role in polio, but  is “far more expensive than vaccination”. Unlike Covid-19, polio and smallpox have no non-human (or environmental) reservoir.

Covid scored high with regard to public and political concern about the burden of disease as well as its social and economic impacts. In contrast, polio had the lowest scores on these parameters among the three diseases.

Smallpox was regarded as “probably the greatest global public investment in human history”. The favourable benefit to cost ratio is still unknown for Covid-19, though it is likely to be favorable.

This preliminary analysis has shown that technically, the feasibility of eradication of Covid-19 was little more than for polio, but it was much less than that for smallpox, suggesting that Covid-19 may be potentially eradicable at some point of time.

But, compared to smallpox and polio, the path to Covid-19 eradication is not without its challenges (technical) viz. vaccine hesitancy, which adversely affects vaccine coverage and emergence of highly transmissible and immune-escape variants of SARS-CoV-2 such as the delta variant, which is a highly infectious variant. It was the key driver of the deadly second wave in the country and has rapidly spread to become the predominant strain now globally. However, in due course of time, the virus may stop evolving to an extent that it may be possible to keep it under control. Persistence of the virus in animal reservoirs is another impediment.

On the other hand, the Covid-19 public health measures of  continuous wearing of 3-layer face masks, hand hygiene, physical distancing, contact tracing, isolation/quarantine are complementary to Covid vaccination. Countries imposed travel bans and closed their borders. Most of the resources were directed towards controlling the onslaught of the pandemic. These highly effective measures give it an advantage over polio and favorably impact its chances of eradication.

The authors write, “the massive scale of the health, social and economic burden from COVID-19 in most of the world means that there is unprecedented global interest in disease control and massive investment in vaccination against the pandemic.”

Smallpox was declared eradicated in 1980 and two out of the three wild poliovirus strains have been eradicated. WPV2 was eradicated in 2015 and WPV3 was declared eradicated in 2019. Wild polio is now found only in Afghanistan and Pakistan and there are occasional outbreaks of circulating vaccine-derived poliovirus (cVDPVs) in some countries of Asia and Africa. And until polio is completely eradicated, there is a risk of resurgence of polio in countries that have now become polio-free. India was declared polio-free in 2014.

We do not know yet how the pandemic will evolve. Will it become endemic (as it appears to be) or will it be eradicated (as this study suggests)? Time will tell.