An online survey of 1,200 previously vaccinated US adults reveals that the most-cited reasons for not getting a bivalent (two-strain) COVID-19 booster dose were lack of awareness about eligibility and overestimations of their own existing immunity.

A team led by Duke University researchers conducted the opt-in survey, published today in Morbidity and Mortality Weekly Report, from Nov 1 to 5, 2022, with an unannounced follow-up survey from Dec 6 to 10.

All respondents had received at least two doses of COVID-19 vaccine, 34.4% had received the bivalent booster, 65.4% were White, 51.9% were women, and the proportions of participants in all age-groups were roughly equal.

The researchers noted that the Centers for Disease Control and Prevention (CDC) first recommended a bivalent booster for people 12 years and older on Sep 1, 2022. As of today, however, only 15.3% of Americans have gotten one.

Good intentions, low follow-through

The 396 participants who received a booster said they did so to protect themselves (90.7%) and others (75.0%) and to prevent severe disease (80.6%).

The most common reasons for not receiving a booster were lack of awareness of eligibility (23.2%) or of vaccine availability (19.3%) and overconfidence in their immunity (18.9%). Over half of those overestimating their immunity had never been infected, and nearly 72% hadn't been infected or received a vaccine dose in the past 6 months.

Health care and public health professionals should use evidence-based strategies ... while also working to increase convenient access.

After participants were given information on eligibility and availability, 67.8% of those who hadn't gotten a booster said that they would pursue one; a survey 1 month later showed that 28.6% of these participants had done so.

Of those who said they would receive a booster but hadn't done so, 82.6% said they still planned to. The most often-cited reasons for not having gotten one were that they were too busy (35.6%), forgot (22.7%), or were worried about side effects (22.7%).

"To help increase bivalent booster dose coverage, health care and public health professionals should use evidence-based strategies to convey information about booster vaccination recommendations and waning immunity, while also working to increase convenient access," the authors wrote.

Several countries have reported instances of H5N1 avian flu infections in mammals, but now Spanish researchers have reported the first known outbreak in farmed mink. They described their investigation yesterday in Eurosurveillance.

The outbreak began in early October 2022 at a farm housing nearly 52,000 mink in Galicia region in northwest Spain. Farm workers noted a sharp increase in deaths, and nasopharyngeal swabbing of the mink revealed H5N1 avian influenza. The mortality rate increased each week, spreading from hot spots to the rest of the barns, peaking toward the end of October.

The mink were housed in partially open barns and were fed raw fish, poultry byproducts sourced from the same region, and other items.

Potential public health implications

A more detailed genetic analysis of the H5N1 virus found that it belonged to the current clade circulating in wild birds and poultry on multiple continents and is most closely related to a strain found in seabirds across Europe.

The scientists found that the H5N1 virus has an uncommon mutation that was seen only once before in a European polecat and that it could have arisen on its own in the mink. They said the mutation in the PB2 gene may have public health implications, given that it is present in the avian-like PB2 gene of the 2009 H1N1 pandemic flu virus and has characteristics that enable recognition by human airway receptor cells.

The virus was involved in wild bird deaths in Galicia in the weeks leading up to the mink farm outbreak, and the birds could have introduced the virus to the farm, given that the barns were partially open. The researchers said, however, that more sequencing is needed to confirm the connection. They added that no avian flu outbreaks had been reported at poultry farms that supplied byproducts to the mink farm.

No related human infections were found at the farm. Spanish mink farm workers are required to wear masks because of the risks of SARS-CoV-2 in those setting. The authors said more studies are under way to look at virulence and transmissibility of the virus. Since mink have been suggested as a possible mixing vessel for respiratory viruses, the group emphasized that interventions are needed to prevent contact between mink and wild animals and to control transmission between farm works and mink.