Invivyd initiates FLINTLOCK Phase 2 trial of VYD2311 in Long COVID
Invivyd has initiated FLINTLOCK, a Phase 2 randomised, double-blind, placebo-controlled trial evaluating its monoclonal antibody candidate VYD2311 in adults aged 18 to 65 years who have Long COVID or post-vaccination syndrome (PVS). The trial, conducted in partnership with the SPEAR Study Group, will enrol approximately 130 participants, with initial data expected in mid-2027.
Participants must have at least two persistent symptoms arising from a confirmed COVID-19 infection or COVID-19 vaccination, present for six months or longer. VYD2311 will be administered as a high-dose intravenous loading infusion on Day 1, followed by intramuscular injections at Day 30 and Day 60. The total dose targets roughly ten times the prophylactic dose being assessed in Invivyd's companion Phase 3 programmes, DECLARATION and LIBERTY, while remaining within a range the company says has already shown an acceptable initial safety profile in first-in-human testing.
The rationale behind the dosing strategy
Invivyd's hypothesis is that both Long COVID and PVS may involve persistent SARS-CoV-2 spike protein driving ongoing inflammation. VYD2311 was engineered to neutralise the spike protein receptor-binding domain and has demonstrated in vitro potency across all SARS-CoV-2 variants tested, from the original Wuhan lineage through contemporary Omicron sub-variants. Marc Elia, chairman and chief executive of Invivyd, said the company believes earlier therapeutic efforts in Long COVID were constrained either by short treatment courses that may have been insufficient to suppress viral replication, or by antibody candidates with narrower variant coverage. The high-dose, multi-injection design of FLINTLOCK is intended to address both limitations.
David Putrino, founding member of the SPEAR Study Group and Nash Family Director of the Cohen Centre for Recovery from Complex Chronic Illness at Mount Sinai, noted that PVS "has received very little attention" from drug developers, framing FLINTLOCK as an important step for a patient population that has largely been without therapeutic options.
Market context and competitive landscape
Long COVID affects an estimated tens of millions of people globally, though precise prevalence figures remain contested in the literature. The condition represents a significant unmet medical need, and the therapeutic landscape remains sparse. Most interventional studies to date have been small and heterogeneous in design. Pfizer's Paxlovid was evaluated in the RECOVER-VITAL trial without demonstrating a significant benefit in Long COVID, and no product has received regulatory approval for the indication to date.
Invivyd's approach of applying a high-dose monoclonal antibody directed at spike protein is mechanistically distinct from antivirals and from the symptomatic or rehabilitative interventions that dominate current clinical practice. The SPEAR Group's involvement lends academic credibility to the trial design, which borrows functional performance endpoints used in prior Long COVID and infection-associated chronic illness studies. However, the condition's heterogeneity and the absence of validated biomarker-based endpoints remain challenges for any Phase 2 proof-of-concept study seeking to produce a clear efficacy signal within a 90-day window.
Invivyd also notes that PEMGARDA (pemivibart), its antibody currently authorised for COVID-19 pre-exposure prophylaxis in immunocompromised patients under a US FDA Emergency Use Authorisation, is not approved or authorised for Long COVID or PVS. The EUA framework supporting PEMGARDA is tied to a pandemic declaration that HHS has indicated will terminate in June 2027, which adds a degree of commercial and regulatory uncertainty to Invivyd's broader pipeline timeline.
Full trial details are expected to be posted to clinicaltrials.gov in the near term. FLINTLOCK sits alongside DECLARATION and LIBERTY within Invivyd's REVOLUTION clinical programme for VYD2311.