A New List of Priority Pathogens that Could Cause the Next Pandemic

On 30 July, WHO published its R&D plan for diseases with epidemic or pandemic potential, prioritising not just individual pathogens but entire families.

The world now has a new, updated list of pathogens it needs to prepare against to prevent future epidemics or pandemics. The list is part of WHO's R&D plan to accelerate the development of medical countermeasures (diagnostic tests, treatments and vaccines) for diseases with epidemic or pandemic potential.

The novelty of this list, compared to those established in 2017 and 2018, is that it focuses on entire families of viruses or bacteria, rather than individual pathogens. In addition, for each family of risk, 'prototype pathogens' are proposed to develop diagnostics, treatments and/or vaccines that could be used for other pathogens in the same family. The aim of this approach is to increase the ability to respond to unknown (the famous disease X) or poorly understood threats.

The result is a larger and more comprehensive list than in 2018, which consisted of a dozen pathogens. The new list includes 34 priority pathogens in 16 families and 30 prototype pathogens in 22 families. In addition, 34 pathogens X have been identified, which currently pose no threat but could cause a pandemic if there are changes in their transmission or virulence.

Pathogen prioritisation at the heart of the R&D plan

The document is the result of a global process in which more than 200 scientists from 50 countries spent two years reviewing the available evidence on 28 viral families and one bacterial group, with a total of 1,652 pathogens, to reach a consensus on which families and pathogens have the greatest epidemic or pandemic potential.

Not all pathogens have pandemic potential

Thousands of viruses and bacteria can infect humans, but only a relatively small number have caused major epidemics or pandemics in history. The working groups used three criteria to prioritise the pathogens that most deserve our attention:

• Transmission: the mode and efficiency of transmission, whether there is asymptomatic transmission, whether we have some immunity to the pathogen, etc.
• Virulence: lethality without treatment, serious sequelae, risk of mutations that increase lethality, etc.
• Countermeasures: the existence - or not - of diagnostic tests, treatments and vaccines to combat the disease.

Twelve viral families of risk, 34 priority pathogens

After two rounds of review and consensus, four virus families (Anellovirus, Papillamovirus, Polyomavirus and Herpesvirus) were discarded as low-risk (i.e. not containing priority pathogens). Out of the remaining 25 families (24 viral, one bacterial) and 185 pathogens, the following were identified:

• 34 priority pathogens (5 of which are bacteria) distributed over 16 families. These are pathogens for which some knowledge already exists and which are known to be capable of causing a public health emergency of international concern (PHEIC). New additions to the 2018 list include for example the plague bacterium and the SARS-CoV-2, monkeypox and several influenza A viruses, because they have already caused health emergencies and the existing countermeasures are inadequate. Many of these pathogens, although currently confined to specific geographical regions, have the potential to spread globally. Some also have the potential for efficient human-to-human transmission.
• 30 prototype pathogens in 22 families considered to be of high or medium risk (i.e. containing at least one priority pathogen). The aim is to use these viruses as a model to develop countermeasures that could be used for other emerging pathogens in the same family, as well as to promote research on lesser-known viruses.
• 34 pathogens X that do not currently pose a threat but could cause a pandemic if plausible changes in their transmission or virulence occur.

Of course, not all of the identified pathogens will cause pandemics, and some that are not on the list may cause serious problems in the future. The experts stress the need to periodically review these lists in the light of global changes, including climate change, urbanisation, deforestation, etc.

The document emphasizes the need to: i) study the biology, transmission and pathogenesis of viral families, especially those considered of high risk because they include several priority pathogens; ii) develop medical countermeasures for known threats (priority pathogens) and potentially similar ones (prototype pathogens); and iii) promote research and development to fight the “inevitable pathogen X”.

The importance of global cooperation

Central to this effort is the imperative of collaboration to ensure global resilience to epidemics and pandemics; including a decentralised collaboration to support research efforts in critical areas of pandemic preparedness and response. The document emphasizes the need to invest in research, development and innovation at the international level. And the importance of ensuring quality, equity in access and trust in the products (vaccines, treatments, diagnostics) resulting from these efforts.

As the scientific community points out, we have never been able to predict what the next emerging pathogen will be. However, this R&D plan, resulting from a global and consensual effort, should enable us to respond better and faster to the next pandemic pathogen, even if today we ignore its identity.