How Long Does It Take Allergy Shots to Work? The 3 Phases of Improvement

In Phase 1 — the first 3 to 6 months of treatment spanning the build-up injections and early maintenance — most patients feel no symptom improvement. This is biologically expected. Phase 1 is the immune priming window: FOXP3+ regulatory T cells become detectable within 2 to 4 weeks, IgG4 begins rising at 1 to 3 months, and basophil sensitivity starts declining. These are real immunological changes measurable in blood tests, but they have not reached the threshold that produces subjective relief. The most important thing to do in Phase 1 is to attend appointments consistently, because missed doses trigger mandatory dose reductions that reset your progress and delay entry into Phase 2.

Phase 2 improvement — typically experienced between months 6 and 12 — is rarely a dramatic shift. Most patients describe it as a gradual reduction in average daily symptom burden: fewer urgent runs to grab an antihistamine, less difficulty sleeping during allergy season, less severe reactions when exposed to their allergen trigger. Clinically, this corresponds to a 34.7 percent reduction in total nasal symptom scores documented in meta-analysis data (Matricardi et al., JACI 2011) and the pooled SMD of -0.73 from 51 randomized controlled trials. The Cochrane data show NNT of 3 for symptom improvement in this phase — meaning roughly 1 in 3 patients has a clear, detectable benefit that clinical measurements would capture.

Stopping during Phase 1 (first 6 months) provides essentially no durable benefit — the immune changes are too early and too fragile to persist after discontinuation. Stopping during Phase 2 (months 6 to 12) may provide some short-term symptom relief that gradually fades, but without completing Phase 3, the disease-modifying tolerance is not established. The EAACI guidelines (Roberts et al., Allergy 2018) explicitly state that at least 3 years of immunotherapy are required for long-term efficacy persisting after discontinuation, and that 2-year courses are insufficient for sustained post-treatment benefit. Stopping in Phase 1 or 2 is the most common reason patients who start allergy shots do not gain the full benefit the treatment is capable of providing.

Evidence suggests 2 years is generally not sufficient for durable post-treatment disease modification. The EAACI guideline (Roberts et al., Allergy 2018) states minimum 3 years are required for long-term efficacy to persist after stopping. Durham et al. (NEJM 1999) based their 3-year post-treatment remission data on patients who completed 3 to 4 years of treatment, not shorter courses. Some patients stopping at 2 years retain partial benefit for a period, but the research on 3 to 12 years of post-treatment benefit refers specifically to patients who completed the full 3 to 5 year maintenance course. Stopping early is not zero benefit, but it is a significant reduction in the duration and magnitude of post-treatment protection.

Evidence from several studies suggests that completing a full immunotherapy course may reduce the risk of developing new allergic sensitizations, though formal randomized trial evidence is limited. Des Roches et al. (JACI, 1997) found 45 percent of SCIT-treated children developed no new sensitizations compared to 0 percent of controls. Purello-D'Ambrosio et al. (Clin Exp Allergy, 2001) reported polysensitization in 23.75 percent of SCIT-treated patients versus 68 percent of untreated monosensitized patients over 4 years. These findings suggest a real protective effect. However, a systematic review by Di Bona (Allergy, 2017) concluded that when restricted to randomized trials, the evidence remains inconclusive — most positive findings come from non-randomized studies. Your allergist can advise whether your specific profile and disease trajectory suggest benefit from extended treatment.

Children who complete all three phases of allergy shot treatment gain an additional benefit not observed in most adult patients: asthma prevention. The PAT (Preventive Allergy Treatment) study — the landmark trial by Moller et al. (JACI, 2002) of 205 children aged 6 to 14 — found that completing 3 years of SCIT significantly reduced new asthma development compared to untreated controls (OR 2.52). At 5-year follow-up, the asthma prevention benefit strengthened (OR 2.68), and at 10-year follow-up (Jacobsen et al., Allergy 2007), 25 percent of treated children had developed asthma versus 45 percent of controls — a benefit that persisted 7 years after treatment ended. This asthma prevention effect reflects the Phase 3 immune remodeling that children's more plastic immune systems respond to particularly well.