Editorials Influenza seasonality: timing and formulation of vaccines Nancy Coxa In all countries with temperate climates, a China, which each have land masses that The information collected by Saha et al.4 clear seasonal peak in influenza activity oc- stretch for thousands of kilometres from clearly demonstrates that country-specific curs during the winter months.1 In countries north to south.6,7 recommendations on influenza vaccination with tropical or subtropical climates, how- WHO’s current recommendations should focus not only on whether the coun- ever, influenza seasonality is more variable for the formulation of the vaccine used try lies to the north or south of the Equator and influenza activity can be observed year- depend primarily on whether the target but also on the number and types of seasonal round – although it often is more intense populations are in the northern or southern patterns in influenza activity that exist within during the rainy seasons.2 hemisphere. Although WHO recommends the country’s borders. Countries with long Effective surveillance is essential for that the choice of formulation to be used in latitudinal spans may need two distinct vac- understanding influenza seasonality and Equatorial areas should be guided by epide- cine policies – one for the temperate areas for determining the best strategies for miological considerations, there has often and another for the more tropical areas – and influenza control. The extensive influenza been too little information available to make to use both the “northern hemisphere” and surveillance network that was established that choice in a rational manner. For the “southern hemisphere” formulations.4,6,7 by the World Health Organization (WHO) northern hemisphere, vaccination typically If appropriate vaccination policies – that during the 1950s has developed into the begins in September of each year and ends in couple the best timing of influenza vaccine Global Influenza Surveillance and Response December or a few weeks later. The vaccine administration with the most recent vac- System. The tropical and subtropical Asian used should be the “northern hemisphere” cine composition – are to be developed, the countries that participate in this system have formulation, which reflects the strains of seasonality of influenza in many countries recently expanded their influenza surveil- virus expected to be in circulation – glob- needs to be better understood. ■ lance. This expansion has revealed not only ally – during the northern winter. For the that the patterns of influenza virus circula- southern hemisphere, vaccination with the References tion in southern and south-eastern Asia are most recent “southern hemisphere” formu- 1. Tamerius JD, Shaman J, Alonso WJ, Bloom- Feshbach K, Uejio CK, Comrie A et al. complex but also that influenza activity in lation typically begins in April and ends in Environmental predictors of seasonal influenza 3 this area may show no marked seasonality. May or a few weeks later. Unfortunately, the epidemics across temperate and tropical climates. The results of a recent analysis of in- varying patterns of seasonality complicate PLoS Pathog 2013;9:e1003194. fluenza surveillance data collected between the optimal timing of influenza vaccinations 2. Moura FE. Influenza in the tropics.Curr Opin Infect 2006 and 2011 in Bangladesh, Cambodia, and the optimal formulation to be used.6 Dis 2010;23:415–20. 3. Bloom-Feshbach K, Alonso WJ, Charu V, Tamerius India, Indonesia, the Lao People’s Demo- As vaccine-induced immunity decreases J, Simonsen L, Miller MA et al. Latitudinal cratic Republic, Malaysia, the Philippines, over time, it is important to vaccinate in the variations in seasonal activity of influenza Singapore, Thailand and Viet Nam are months before epidemic peaks of influenza.8 and respiratory syncytial virus (RSV): a global published in this issue of the Bulletin of the Saha et al. demonstrate that influenza activ- comparative review. PLoS ONE 2013;8:e54445. World Health Organization.4 Although this ity in most northern hemisphere countries 4. Saha S, Chadha M, Al Mamun A, Rahman M, Strum-Ramirez K, Chittaganpitch M et al. analysis revealed year-round circulation of that lie in tropical Asia peaks between July Influenza seasonality and vaccination timing in influenza viruses in all 10 study countries, and October – or several months earlier the tropical and subtropical areas of southern two main patterns of influenza seasonality than in the countries situated in temperate and south-eastern Asia. Bull World Health Organ were also evident.4 In Indonesia, Malaysia areas of the northern hemisphere.4 In tropi- 2014;92:318–330. and Singapore – i.e. the study countries that cal Asia, therefore, influenza vaccinations 5. Western Pacific Region Global Influenza Surveillance and Response System. lie on or close to the Equator – there was no offered in September–December are pro- Epidemiological and virological characteristics obvious discrete peak in influenza activity. vided too late and those vaccinated at this of influenza in the Western Pacific Region of the In the whole or part of each of the seven time would be expected to have declining World Health Organization, 2006–2010. PLoS ONE other – more northern but still at least par- vaccine-induced antibody titres during the 2012;7:e37568. 6. de Mello WA, de Paiva TM, Ishida MA, Benega tially tropical – study countries, influenza next seasonal peak of influenza activity, in MA, Dos Santos MC, Viboud C et al. The dilemma activity showed a distinct peak between July–October of the following year. Tropical of influenza vaccine recommendations when July and October, in the summer monsoon areas in southern and south-eastern Asia applied to the tropics: the Brazilian case season. This peak contrasted with the winter should consider vaccination – with the examined under alternative scenarios. PLoS ONE peak that typifies influenza seasonality in the “southern hemisphere” vaccine formulation 2009;4:e5095. 3,5 7. Yu H, Alonso WJ, Feng L, Tan Y, Shu Y, Yang W more temperate areas of the world. As a – in May–June of each year. et al. Characterization of regional influenza latitude of about 30°N appeared to mark the While strategies for influenza vaccina- seasonality patterns in China and implications for northern limit of influenza with a summer tion are well advanced in many temperate vaccination strategies: spatio-temporal modeling peak in activity, some study countries – such areas, data to support the timing of vac- of surveillance data. PLoS Med 2013;10:e1001552. as India – had influenza with a summer peak cination efforts in tropical areas of Asia 8. Kissling E, Valenciano M, Larrauri A, Oroszi B, Cohen JM, Nunes B et al. Low and decreasing in the south but influenza with a winter peak have been quite limited. For this and other vaccine effectiveness against influenza A(H3) in the north. A similar relationship between reasons, many countries in tropical Asia use in 2011/12 among vaccination target groups latitude and the timing of peak influenza little or no influenza vaccine – despite their in Europe: results from the I-MOVE multicentre activity has been observed in Brazil and considerable burdens of influenza disease. case-control study. Euro Surveill 2013;18:pii20390. a Influenza Division, Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta, GA 30333, United States of America. Correspondence to Nancy Cox (e-mail: [email protected]). Bull World Health Organ 2014;92:311 | doi: http://dx.doi.org/10.2471/BLT.14.139428 311.