Influenza: seasonality and travel-related considerations

Abstract

Rationale for review: This review aims to summarize the transmission patterns of influenza, its seasonality in different parts of the globe, air travel- and cruise ship-related influenza infections and interventions to reduce transmission.

Key findings: The seasonality of influenza varies globally, with peak periods occurring mainly between October and April in the northern hemisphere (NH) and between April and October in the southern hemisphere (SH) in temperate climate zones. However, influenza seasonality is significantly more variable in the tropics. Influenza is one of the most common travel-related, vaccine-preventable diseases and can be contracted during travel, such as during a cruise or through air travel. Additionally, travellers can come into contact with people from regions with ongoing influenza transmission. Current influenza immunization schedules in the NH and SH leave individuals susceptible during their respective spring and summer months if they travel to the other hemisphere during that time.

Conclusions/recommendations: The differences in influenza seasonality between hemispheres have substantial implications for the effectiveness of influenza vaccination of travellers. Health care providers should be aware of influenza activity when patients report travel plans, and they should provide alerts and advise on prevention, diagnostic and treatment options. To mitigate the risk of travel-related influenza, interventions include antivirals for self-treatment (in combination with the use of rapid self-tests), extending the shelf life of influenza vaccines to enable immunization during the summer months for international travellers and allowing access to the influenza vaccine used in the opposite hemisphere as a travel-related vaccine. With the currently available vaccines, the most important preventive measure involves optimizing the seasonal influenza vaccination. It is also imperative that influenza is recognized as a travel-related illness among both travellers and health care professionals.

Keywords: Respiratory virus; antiviral; cruise ship; outbreak; screening; transmission; vaccination.

Figure 1

Influenza seasonality around the globe based on data from the WHO^, and other references^,. There are 2 areas with anomalous distribution of influenza seasonality. The first is located in Africa, where based on the WHO classification, the Western Africa influenza vaccination zone has a SH seasonality, whereas Equatorial Africa influenza vaccination zone has an NH seasonality, despite being located to the south of Western Africa influenza vaccination zone. As a result, some countries with a reported NH seasonality are surrounded by countries with SH seasonality. The second is in the Indo-Australian Archipelago, where islands shared between Indonesia and other nations (Papua New Guinea, Malaysia, Brunei, Timor-Leste) appear to have different influenza seasonality from East to West. Unlike the surrounding nations, Indonesia is reported as having a NH seasonality.