Vaccination remains the most effective strategy for avian influenza prevention and control in humans, despite varying vaccine efficacy across strains.
With cows, cats and dairy workers in the firing line of the new strain of bird-flu to emerge in the US, HN51, researchers have begun the task of evaluating the current capabilities of vaccines to protect against transmission within a human population.
Published in Human Vaccines & Immunotherapeutics, the results are timely following news on 22 May that the bird flu strain H5N1 had, for a second time, jumped from cattle in America to a human, prompting fears of subsequent human-to-human infection.
Instances of the avian influenza were first recognised in US cattle in March and since then, the strain has mainly spread within cattle herds, resulting in the further discovery of very high levels of virus in raw milk – proving lethal to numerous farm cats.
Following tests on the first human instance, it was seen that the strain had mutated to be better adapted to mammalian cells, but so long as that human didn’t pass it onto another person, it likely stopped the spread at that point.
With the second case, the CDC says it has been monitoring influenza surveillance systems, especially in impacted states. “There has been no sign of unusual influenza activity in people, including in syndromic surveillance,” they report.
Lead author Dr Flavio Cargnin Faccin, from the University of Georgia, USA, explained that the concern now is that if H5N1 continues to mutate in close quarter cattle farms – and this continues long enough – it has the potential to find a combination that will easily spread to humans.
“The H5N1, H7N9, and H9N2 subtypes of avian influenza virus pose a dual threat, not only causing significant economic losses to the global poultry industry but also presenting a pressing public health concern due to documented spillover events and human cases,” Dr Faccin said.
“This deep delve into the landscape of avian influenza vaccines for humans shows vaccination remains the primary defence against the spread of these viruses.”
While live attenuated influenza vaccines (LAIVs) are known to induce a wider immune response than inactivated vaccines, including antibody production, and mucosal and cellular defences, the authors suggested this broader response may offer greater protection – though further research is needed.
Modern alternatives, such as virus-like particle (VLP) vaccines and messenger RNA (mRNA) vaccines, that have emerged more recently. Although VLP vaccines for bird flu have limited clinical trial data in humans, results from studies in mice and ferrets showed promise.
“mRNA vaccines against H5N1 and H7N9 bird flu subtypes also generated a rapid and strong immune response in mice and ferrets, and, while data in humans is scarce, results from a phase 1 study of an H7N9 mRNA vaccine in healthy humans were encouraging,” Dr Faccin said.
“Overall, exploring and employing a diverse range of vaccine platforms, will be crucial for enhancing pandemic preparedness and mitigating the threat of avian influenza viruses.”