We were still dealing with COVID-19 when another viral infectious disease reared its head. At the end of May 2022, hundreds of cases of monkeypox (Mpox) were reported in dozens of countries in Africa, the Middle East, Europe, and North and South America.

Mpox is not new; the disease is endemic in parts of West and Central Africa. However, the new cases seem to be predominantly occurring among men who have sex with men. This is noteworthy because until recently, it wasn't entirely clear whether Mpox could be sexually transmitted. Research in Africa had consistently shown that Mpox was not very contagious, which meant that past outbreaks typically died out on their own.

But in contrast to previous outbreaks, this time, many of the infected individuals had few, atypical, or even no symptoms. This had a significant impact on the virus's spread, as indicated by our models based on behavioral data from Belgian men who have sex with men. We calculated that contact tracing could reduce the epidemic, even if only 10% of all traced contacts effectively discontinued their sexual activities. According to the same model, it would be more beneficial to proactively vaccinate individuals with a high-risk profile rather than only vaccinating identified partners after possible exposure.

The Mpox outbreaks illustrate a new epidemic or pandemic will not necessarily be triggered by a new virus. Apart from the global spread of the Mpox virus, measles are on the rise in India. There have been massive outbreaks in several major cities after millions of children were not vaccinated on time. In our country, healthcare workers have their hands full with RSV infections in young children during the third winter after COVID-19. New outbreaks of bird flu have occurred worldwide, and in Africa, the Marburg virus has reappeared in 2023.

At any moment, a new variant of an existing bacterium or virus can emerge that makes the disease more contagious or severe. Most epidemiologists had feared a new influenza pandemic sooner or later, but this time it was a coronavirus that gripped the world. In other words, we must be prepared for anything.

Pathogen X

To make matters worse, our current knowledge of viruses and bacteria is very limited. Scientists have only studied a relatively small number of species that make us sick or have some kind of impact on our lives. We still know very little about many others. Some researchers estimate that there are over 1.5 million unknown viruses that infect mammals and birds! While this number is contested, there's no doubt that we are dealing with an enormous number of unknown potential pathogens.

The World Health Organization keeps a close watch on a few known viruses and diseases. COVID-19, MERS, and SARS, of course, but also Zika and Ebola are on that list. A new variant or even just relaxed attention or measures could lead to serious epidemics or even pandemics again. Additionally, the emerging antibiotic resistance of various formerly treatable bacteria poses an imminent threat.

Geneticists keep watch

How do you prepare when you can't predict where the danger will come from? By keeping an eye on pathogens with a wide view and remaining vigilant for new species and variants. Scientists do this by routinely analyzing the genomes of bacteria and viruses. This allows them to monitor the origin, spread, and characteristics of a new pathogen.

Genetic epidemiology was found to make a significant difference in quickly understanding how both Ebola and Zika spread and evolved. When the coronavirus emerged, sequencing results were produced and shared on an unprecedented scale and speed. New variants were detected in no time and incorporated into epidemiological models to predict their global spread and impact. This is good news, but we can do even better.

Especially in the early phase of the pandemic, it was not straightforward to establish reliable sequencing programs everywhere to monitor the virus's spread in a representative manner. In addition to the right expertise for DNA (or RNA) analysis, sufficient time and resources are needed to subsequently analyze the obtained data. Some countries have processed a remarkable number of samples, but the results were not analyzed in a timely manner. This can unjustifiably lead policymakers to conclude that investments in sequencing were a waste and should be cut in the future. There is an urgent need for international guidelines indicating where, when, and how many samples should be taken and analyzed to obtain and exchange reliable information about the spread of a new pathogen in different phases of an epidemic or pandemic.

The impact of aging

A pathogen can mutate, but society can change as well. The ongoing aging of the population leads to an altered population pyramid. The associated shift towards more single individuals and more people in collective housing arrangements such as nursing homes can further exacerbate the consequences of future SARS-CoV-2 or influenza outbreaks. Therefore, we simulated how the expected changes in the age structure of the Belgian population between now and 2050 will affect the disease burden of future epidemics.

We simulated COVID-19 and influenza outbreaks on different scales and found that as the proportion of older people in society increased, the attack rate (the proportion of susceptible people who become infected) decreased slightly because older people have fewer social contacts and therefore are less likely to acquire and transmit infections. But even though the attack rate per person was lower, the estimated disease burden was higher because the course of the disease and the risk of death are much greater in older people.