Influenza remains a significant public health concern globally, causing seasonal outbreaks that can strain healthcare systems and lead to increased hospitalizations and mortality. Each year, the flu virus undergoes changes, requiring vigilant monitoring to track its spread and evolution. In 2024, flu surveillance has continued to play a crucial role in guiding public health interventions and vaccine formulations, particularly in regions like South Africa, where influenza activity follows a predictable seasonal pattern. The following is a comprehensive report on flu surveillance in South Africa for 2024.
In 2024, influenza activity in South Africa followed a typical seasonal pattern, beginning in April and peaking in early June. As expected during the winter months, influenza transmission levels reached moderate to high intensity. The flu season was dominated by the influenza A(H1N1)pdm09 strain, but a resurgence of influenza B was also detected in mid-July. Despite some genetic differences between circulating viruses and the Southern Hemisphere vaccine strain, the vaccine has remained effective.
The World Health Organization’s National Influenza Centre (NIC), housed at the NICD’s Centre for Respiratory Diseases and Meningitis, monitored the flu season closely. This surveillance report, for weeks 1 through 33 of 2024, highlights the importance of continuous tracking and genetic analysis of influenza strains to ensure effective public health responses.
This year’s flu season underscored the significance of vaccination and early detection. The continued surveillance by the NIC ensures that the country can adapt its flu vaccination strategies and maintain preparedness for future influenza outbreaks, helping to mitigate severe outcomes and protect vulnerable populations. With the potential for influenza to evolve and spread unpredictably, sustained monitoring and timely public health interventions remain critical.
Highlights
Epidemiology
- Influenza activity was observed from weeks 1 through 33, with an overall detection rate from 1 January through 18 August 2024 of 15.9% (887/5590). Using the Moving Epidemic Method (MEM) the levels of activity reached moderate and high levels in the ILI and Pneumonia Surveillance programmes, respectively. Influenza single infections where a subtype/lineage could be determined were dominated by A(H1N1)pdm09 (67.4%, 580/860) followed by B/Victoria (30.6%, 263/860). Low numbers of influenza A(H3N2) (1.9%, 17/860) were detected. Influenza B/Yamagata was not detected.
- Almost all of the South African 2024 influenza A(H1N1)pdm09 viruses were subclade 6B.1A.5a.2a (5a.2a)
(203/205, 99.0%), with two additional substitutions, T120A and K169Q, compared to the 2023 viruses. The 2024 Southern Hemisphere A(H1N1)pdm09 vaccine strain (A/Victoria/4897/2022) is a subclade 5a.2a.1 virus, which differed to the subclade of the viruses circulating in South Africa in 2024. However, antigenic analysis of South African viruses showed that all (32/32, 100%) were well inhibited by subclade 5a.2a.1 vaccine-type antisera (A/Victoria/4897/2022 and A/Wisconsin/67/2022). Phenotypic neuraminidase inhibitor susceptibility testing indicated that one (1/26, 3.8%) A(H1N1)pdm09 virus showed highly reduced inhibition to oseltamivir and paramivir, this virus was genotypically confirmed to contain the H275Y substitution in neuraminidase. - Six influenza A(H3N2) viruses circulating in South Africa in 2024 were sequenced, and all (6/6, 100%) belonged to clade 3C.2a1b.2a.3a.1 (2a.3a.1) together with the 2024 Southern Hemisphere vaccine strain (A/Thailand/8/2022). None of the A(H3N2) viruses were found to be phenotypically (n=2) or genotypically (n=6) resistant to neuraminidase inhibiting antivirals.
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