**6. Conclusions**

When comparing major influenza and coronavirus pandemics in recent history, it becomes clear that their similarities are not mere coincidences but seem to be somewhat inherent to how and why certain pathogens are easily transmitted from person to person and usually by multiple modes of transmission. In the past 100 years, all major pandemics have been caused by respiratory viruses that emerged during the winter season in the Northern Hemisphere during the second half of the normal flu season. They have all originated from a non-human source and most have exhibited the ability to cross from humans back into certain animal populations serving as the site for antigenic shifts or genetic recombination events. While there is no guarantee all future pandemics will follow these trends, understanding these animal to human transmission patterns may assist us in preparing for future emerging infections. Pandemics tend to occur in at least two consecutive waves of transmission. The impact of each wave depends on the underlying level of immunity of the population and community mitigation interventions. In every pandemic, there are specific age groups at risk of developing severe disease and dying. Most of these outcomes are the result of large pools of immunologically naïve populations, immunosenescence of the elderly, and a high prevalence of medical comorbidities. Among many influenza pandemics, school-age children were major drivers of household transmission. The COVID-19 pandemic has been associated with a high-attack rate

among household contacts. Therefore, the prompt institution of mitigation interventions including non-pharmaceutical interventions such as social distancing, travel restrictions, interruption of mass gatherings, and community quarantine is critical to achieving at least a 50% reduction in transmission, which correlates with a basic reproductive number lower or equal to one (R0 ≤ 1).

The development of vaccines is a vital part of any pandemic response. However, in all cases starting with the grea<sup>t</sup> influenza pandemic of 1918–1919, the timeline for vaccine development, testing, approval, and production has been inadequate to have a substantial e ffect in mitigating the spread and impact of each pandemic. The first 2–3 waves are most likely to occur within 12–15 months of the virus originating and therefore it is crucial to focus on mitigating interventions targeting infection prevention strategies [12]. Indeed, it is important to allocate the bulk of our public health e fforts to non-pharmacologic interventions and to the medical care of symptomatic cases to reduce fatalities until safe and e fficacious vaccines are fully developed and distributed. Another important aspect to consider during a pandemic is maintaining adequate coverage levels of routine childhood vaccination in order to prevent the occurrence of outbreaks of vaccine-preventable diseases.

Given the degree of globalization and interconnectivity of modernity, pandemics remain as perennial threats for human societies. We can never be fully prepared for future pandemics. However, given that pandemics tend to disproportionately impact socially disadvantaged populations, there is an important urgency to continue addressing health inequities and structural social vulnerabilities that many people globally endure.

**Author Contributions:** G.M., Z.S., C.F.-P., J.S. drafted the initial manuscript and reviewed the literature. Edited and approved the last version. A.F.H.-M., P.C., T.W. reviewed the literature, edited, and approved the last version. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
