When the researchers measured levels of SARS-CoV-2 spike protein — the part of the virus that enables it to penetrate and infect host cells and what COVID-19 vaccines use to trigger immune responses against the virus — they found that some individuals with PVS, even those without evidence for infection, had higher levels of spike protein than controls. Typically spike protein can be detected for a few days after vaccination, but some participants with PVS had detectable levels more than 700 days after their last vaccination. Persistent spike protein has been associated with long COVID as well.