The emergence and re-emergence of pathogens is a public-health concern, which has become more evident after the coronavirus disease 2019 (COVID-19) pandemic and the monkeypox outbreaks in early 2022. Given that vaccines are the more effective and affordable tools to control infectious diseases, the authors reviewed two heterologous effects of vaccines: the trained immunity and the cross-reactivity. Trained immunity, provided by attenuated vaccines, was exemplified in this article by the decreased the burden of COVID-19 in populations with high Bacille Calmette-Guerin (BCG) coverage. Cross-reactive responses were exemplified here by the studies which suggested that vaccinia could help controlling the monkeypox outbreak, because of common epitopes shared by orthopoxviruses. Although modern vaccination is likely to use subunit vaccines, the authors discussed how adjuvants might be the key to induce trained immunity and improve cross-reactive responses, ensuring that heterologous effects would improve the vaccine’s response.