Alum salts have been used for more than six decades to improve the effectiveness of vaccines but scientists continue to discover new things about their properties.
New research by a team from the University of Keele suggests some forms have the potential to be more toxic than others. The discovery could help guide the selection of adjuvants by vaccine formulators but also leads the researcher to recommend that alum should not be used as a placebo control in vaccine trials.
Writing in the journal Nature Scientific Reports, Professor Chris Exley and colleagues describe a possible mechanism by which aluminium could be transported into the central nervous system.
The discovery means that "all vaccine trials which use aluminium salts as adjuvants must not use the aluminium adjuvant as the control or placebo," according to Prof Exley, who has been researching aluminium toxicology for since the mid-1980s.
The safety of alum has been under scrutiny for years, with suggestions that alum-based adjuvants can cause autoimmune reactions and brain encephalopathies.
However, regulatory authorities around the world - including the European Medicines Agency (EMA) and US Food and Drug Administration (FDA) as well as the World Health organization (WHO) - have so far concluded it is safe and effective based on the evidence available to date.
Amid growing concern about declining levels of vaccination against some disease, the FDA conducted a review of alum in infants and reported last year that it did not reveal any significant cause for concern other than injection site reactions.
Now, the Keele team says it has demonstrated that one alum salt - aluminium hydroxyphosphate (Adju-Phos) - is more likely to cause injection site reactions than another form called aluminium oxyhydroxide (Alhydrogel). Both are commonly used in vaccine formulations.
When they looked at the particle size distribution (PSD) of the two adjuvants, the scientists fund that aluminium oxyhydroxide was much more likely to be taken up into white blood cells, which means it could be transported around the body via lymph nodes.
This loading of aluminium into viable cells" offers a mechanism whereby significant amounts of aluminium, a known neurotoxin, might be translocated throughout the body and even across the blood brain barrier and into the central nervous system," they suggest.
Prof Exley noted that there are no clinically-approved aluminium adjuvants only clinically approved vaccines which use aluminium adjuvants.
"This has been common practice for many years and has resulted in many vaccine-related adverse events due in part or in entirety to aluminium adjuvants being unaccounted for in vaccine safety trials."