One of the largest public health crisis in modern history is 1918 influenza pandemic “Spanish Flu”, which marks 100th anniversary in 2018. Post first pandemic three influenza pandemics occurred during the 20th century. For the new influenza A virus (1) early vaccine development spurred (2,3) . Then in Asia (2004) reemerged avian influenza A(H5N1) which served as a reminder of this threat. This reemergence brought about an acceleration of national and international efforts to prepare for the next pandemic (4).

WHO has put efforts to expand influenza surveillance and laboratory capacity. Undoubtedly it has contributed to a more effective response during the 2009 influenza A(H1N1) pandemic (5). WHO held a Global Influenza Program in November 2009, to assess the status of global research efforts to understand, prevent, and control influenza. A framework for research agenda addressing gaps in our understanding of the emergence, transmission, mitigation, and management of influenza was launched. The agenda is a broad‐based public health research strategy for influenza, offering high‐priority research recommendations organized around a framework of five key research areas or “streams” of particular importance to public health decision‐makers, especially those in less‐resourced countries:

Stream 1: Reducing the risk of emergence of pandemic influenza.

Stream 2: Limiting the spread of pandemic, zoonotic, and seasonal epidemic influenza.

Stream 3: Minimizing the impact of pandemic, zoonotic, and seasonal epidemic influenza.

Stream 4: Optimizing the treatment of patients.

Stream 5: Promoting the development and application of modern public health tools.

There is a differences among seasonal, pandemic, and zoonotic or variant influenza. Seasonal influenza viruses circulate and cause disease in humans every year. There are three large groupings or types of seasonal influenza viruses, labeled A, B, and C. Pandemic influenza occurs when an influenza virus which was not previously circulating among humans and to which most people don’t have immunity emerges and transmits among humans. Some pandemics may result in large numbers of severe infections while others will result in large numbers of milder infections, but the reasons behind these differences are not completely understood. The most notorious pandemic was the “Spanish Flu” in 1918-1919 which caused an estimated 20-50 million deaths worldwide. Subsequent pandemics in 1957 and 1968 resulted in many fewer deaths in spite of large portions of the world’s population being susceptible to infection. In 2009, a strain of influenza A(H1N1) virus which had not ever been seen before, emerged, This pandemic A(H1N1)2009 virus has been widely circulating across the globe since 2009. It is now established in human populations as a seasonal influenza virus. Currently there is no longer a pandemic virus circulating in the world. Zoonotic or variant influenza are where humans get infected with influenza viruses that are routinely circulating in animals. Few examples are avian influenza virus subtypes A(H5N1) and A(H9N2) and swine influenza virus subtypes A(H1N1) and (H3N2). Over the past decades, there have been multiple instances of sporadic transmission of influenza viruses between animals and humans. When viruses of subtype A(H3N2) circulating in swine, began to infect people in the USA in 2011, they were labeled “variant” (with a “v” placed after the name of the virus) in order to distinguish them from human viruses of the same subtype1. Other animal viruses, e.g. avian influenza A(H5N1), A(H7N7), A(H7N9), and A(H9N2), infecting people are simply called “avian influenza2 ” or “zoonotic influenza” viruses.

WHO convenes technical consultations in February and September each year to recommend viruses for inclusion in influenza vaccines for the northern and southern hemisphere influenza seasons, respectively. In the temperate zone of the northern hemisphere, influenza activity declined from February to April and remained at inter-seasonal levels in most countries. Influenza A(H1N1)pdm09 was the predominant type A in most countries in Europe, northern and western Africa and Asia. Influenza A(H3N2) was the predominant type A in northern Europe, North America and some countries in Asia. Influenza B viruses circulated in higher proportions than influenza A viruses in many countries in Europe. Influenza activity in northern Africa was high in several countries in February and March with widespread A(H1N1)pdm09 outbreaks in Algeria and A(H3N2) in Morocco. Influenza activity in the tropical and subtropical region of Asia was high with regional/widespread outbreaks reported in South-East Asia. Influenza activity in tropical regions of South America was generally high with A(H1N1)pdm09, A(H3N2) and B outbreaks reported. In the temperate zone of the southern hemisphere, influenza activity increased from March to June. In the southern cone of South America there was co-circulation of influenza A and B viruses, and in South Africa A(H1N1)pdm09 virus predominated with regional activity of influenza B virus reported later in the winter season. Influenza activity was low in Australia and New Zealand throughout this period.  Between February and September 2018, influenza activity was reported globally, with influenza A(H1N1)pdm09, A(H3N2) and influenza B viruses co-circulating. Over the last decade low and middle income countries of tropics and subtropics have also introduced or expanded seasonal influenza vaccination in their national immunization program. The two challenging questions to these countries are when to vaccinate and which formulation (northern or southern hemisphere) due to the complexities of seasonal influenza activity in tropics and subtropics e.g., multiple peaks and identifiable year-round activity.

The WHO Global Influenza Program recommended an evidence-based practical approach in order to group countries sharing the same seasonality pattern and virus antigenic characteristics. WHO says that from an antigenic evolution perspective, there is no evidence to suggest the need for a 3rd recommendation for vaccine composition specifically for the tropics and subtropics. It further says that the most recent WHO influenza virus vaccine recommendation should be used, independent of the hemisphere in which the country is situated. Countries are encourage to strengthen surveillance and share viruses with WHO Collaborating Centers of Global Influenza Surveillance and Response System for analysis of antigenic and genetic similarity with both the NH and SH vaccine formulation of the respective season, to determine which vaccine formulation has a better antigenic correspondence with currently circulating viruses.

On 27 September 2018 WHO issued “Recommended composition of influenza virus vaccines for use in the 2019 southern hemisphere influenza season”. It is recommended that egg based quadrivalent vaccines for use in the 2019 southern hemisphere influenza season contain the following:

– an A/Michigan/45/2015 (H1N1)pdm09-like virus;

– an A/Switzerland/8060/2017 (H3N2)-like virus;

– a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage); and

– a B/Phuket/3073/2013-like virus (B/Yamagata/16/88 lineage). I

It is recommended that egg based trivalent vaccines for use in the 2019 southern hemisphere influenza season contain the following:

– an A/Michigan/45/2015 (H1N1)pdm09-like virus;

– an A/Switzerland/8060/2017 (H3N2)-like virus; and

– a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage).

It is recommended that the A(H3N2) component of non-egg based vaccines for use in the 2019 southern hemisphere influenza season be an A/Singapore/INFIMH-16-0019/2016-like virus together with the other vaccine components as indicated above.

The study of previous pandemics help to guide current and future pandemic planning. It can lead to a better understanding of the complex ecobiology that underlies the formation of pandemic strains of influenza viruses.


  1. Monto A, Sellwood  History and epidemiologic features of pandemic influenza. In: Van-Tam J, Sellwood C, editors. Pandemic influenza. Boston: CABI; 2013. p. 40–9.
  2. Dowdle Influenza immunoprophylaxis after 30 years’ experience In: Nayak DP, editor. Genetic variation among influenza viruses. New York: Academic Press, Inc.; 1981. p. 525–34.
  3. Commission on Influenza. A clinical evaluation of vaccination against influenza. JAMA. 1944;124:982–5.
  4. Nguyen-Van-Tam JS, Bresee  Pandemic preparedness and response. In: Webster RG, Monto AS, Braciale TJ, Lamb RA, editors. Textbook of influenza. Sussex (UK): John Wiley and Sons; 2013. p. 453–69.