Updated
November 18th, 2019

Southern Hemisphere Influenza Season Was Peculiar

Influenza seasonality and vaccine effectiveness varied substantially within the southern hemisphere during 2019

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A new report from Eurosurvellance shows that within countries of the Southern Hemisphere, the timing, duration, and intensity of the predominant circulating influenza viruses all varied during the 2019 influenza season.

This variability was noted even between neighboring countries.

This Eurosurveillance report on November 7, 2019, includes data from Australia, Chile, New Zealand, and South Africa.

This means that the ‘flu activity in one country was not indicative of activity in another country, even when influenza seasons are contemporaneous,’ concluded this November 7, 2019 report.

The early Vaccine Effectiveness (VE) estimates for the 2019 influenza season in the Southern Hemisphere presented in this report were highest for influenza A(H1N1)pdm09 and lowest for A(H3N2).

However, the utility of these estimates for the Northern Hemisphere may be limited because the 2019 Southern Hemisphere vaccine differed from the 2019/20 Northern Hemisphere formulation in 3 of 4 components, A(H1N1)pdm09, A(H3N2) and B/Victoria. 

Nevertheless, these estimates or earlier versions of them were included with other data reviewed at the WHO Consultation and Information Meeting on the Composition of Influenza Virus Vaccines for Use in the 2020 Southern Hemisphere Influenza Season.

There are many potential sources for this heterogeneity that affect not only the VE estimates but interpretation of weekly activity rates.

  • First, with random sampling, we should not expect estimates to be the same.
  • Second, when samples are small they may be vulnerable to statistical biases, such as sparse data bias, and bias due to measurement errors may be more profound.
  • Third, there were many differences in study design. Case ascertainment differed; for example, a SARI case definition was used in New Zealand and Chile, but not in Australian hospital surveillance. Exposure ascertainment also differed, with varying availability of registries to verify vaccination status and the use of different vaccines. In particular, the adjuvanted vaccines used among Australians ≥ 65 years of age might be expected to yield higher VE than standard vaccines.
  • Fourth, vaccine coverage varied. Low vaccination coverage, as observed in South Africa, affects power and precision and can exacerbate the bias induced by measurement errors. Higher coverage, as seen in Chile and among elderly patients in New Zealand and Australia, may mean that many more people in the sample are repeat vaccinees. Repeat vaccination may negatively impact VE and could result in lower VE estimates in highly vaccinated populations.
  • Fifth, although only limited virological data were available, we observed differences in circulating A(H3N2) virus clades and B lineages. This may impact both seasonality and VE, particularly as most A(H3N2) viruses sequenced appeared to be in different clades from the vaccine virus (3C.2a2). Notably, most A(H3N2) viruses were also in different genetic groups from the 2019/20 northern hemisphere vaccine.

In conclusion, this report attempted to briefly summarise and interpret the 2019 influenza season in 4 Southern Hemisphere countries and have presented early VE estimates. 

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We observed substantial variation in available data on influenza seasonality and VE within the southern hemisphere in 2019, which is unsurprising given the many differences in surveillance among these countries.

Flu vaccine news

The US Centers for Disease Control and Prevention (CDC) suggests everyone 6 months of age and older should get a flu vaccine every season, with rare exceptions.

Different flu vaccines are approved for use in different groups of people. There are flu shots approved for use in children as young as 6 months of age and flu shots approved for use in adults 65 years and older.

Additionally, the CDC continues to recommend that pregnant women get an influenza vaccine during any trimester of their pregnancy because influenza poses a danger to pregnant women and a flu shot can prevent influenza in pregnant women.

As always, vaccination decisions should be part of an ongoing discussion between a provider and patient.

NOTE:  Caution should be applied when attempting to infer the impending Northern Hemisphere influenza season based on these observations.

Various potential conflicts of interest were disclosed. 

Influenza Vaccine news published by Precision Vaccinations