Immunotherapy for Allergy: SCIT, SLIT, and Emerging Options Compared

Allergy shots are one specific form of allergy immunotherapy. The full terminology tree is: immunotherapy (broad category including cancer immunotherapy) contains allergen immunotherapy (AIT, treatments for IgE-mediated allergy), which contains subcutaneous immunotherapy (SCIT — allergy shots) and sublingual immunotherapy (SLIT — drops or tablets). When your allergist says 'immunotherapy,' they mean allergen immunotherapy. When a cancer specialist says 'immunotherapy,' they mean checkpoint inhibitors or CAR-T therapy — a completely different mechanism. The World Health Organization officially endorsed the term 'allergen immunotherapy' in 1998 (Bousquet et al., JACI 1998) to distinguish it from cancer treatments.

Both achieve broadly comparable symptom reduction in the best available evidence. The most rigorous network meta-analysis (Nelson et al., JACI Pract 2015) found no statistically significant efficacy difference between SLIT tablets and SCIT for grass pollen. Earlier indirect comparisons (Di Bona 2012; Dretzke 2013) suggested SCIT may have a modestly larger effect, but head-to-head double-blind trials with adequate power do not exist for most allergens. For grass, birch, and house dust mite, both routes have strong evidence. For cat, mold, and some tree pollens, SCIT has a deeper evidence base. The safety advantage of SLIT is clear: zero documented fatalities versus allergy shots' historical rate of approximately 1 fatal reaction per 2.5 million injections.

Four SLIT tablets are FDA-approved as standardized biologics in the United States: Grastek (Timothy grass, approved 2014, ALK), Ragwitek (short ragweed, approved 2014, ALK), Oralair (5-grass mix, approved 2014, Stallergenes), and Odactra (house dust mite, approved 2017, ALK/Merck; expanded to children 5-11 in February 2025). All four treat only one allergen or allergen group each — a limitation for the estimated 60% or more of patients who are sensitized to multiple allergens. Palforzia (peanut SLIT, approved 2020) is not an environmental allergy treatment but uses the same sublingual delivery concept. Compounded SLIT drops treat multiple allergens in a single formulation but are not FDA-approved.

A minimum of 3 years is required for lasting post-treatment benefit from both SCIT and SLIT. The GRASS trial (Scadding et al., JAMA 2017) showed that 2-year courses of either SCIT or SLIT produced no significant sustained benefit at 1 year post-treatment — establishing 3 years as the evidence-based minimum. Durham et al. (NEJM 1999) demonstrated that 3-4 years of grass pollen SCIT produces clinical remission persisting at least 3 years after stopping. EAACI guidelines (Roberts et al., Allergy 2018) formally recommend a minimum of 3 years. Many allergists recommend 4-5 years for patients with severe or persistent symptoms.

Evidence from landmark trials suggests immunotherapy can reduce the risk of asthma development in children with allergic rhinitis. The PAT study (Möller et al., JACI 2002) followed 205 children with grass or birch pollen rhinitis: those who received 3 years of SCIT were significantly less likely to develop asthma, with an odds ratio of approximately 2.5 favoring the treated group. This protective effect persisted at 10-year follow-up (Jacobsen et al., Allergy 2007), with asthma developing in 25% of treated children versus 45% of untreated controls. SLIT also shows asthma prevention signals in the GAP trial, though the primary endpoint was not met. This disease-modification potential is unique to immunotherapy — medications do not prevent asthma development.

Intralymphatic immunotherapy (ILIT) delivers allergen directly into inguinal lymph nodes under ultrasound guidance — just 3 injections over 8 weeks achieve immune changes similar to years of conventional SCIT (Senti et al., PNAS 2008). It remains investigational and is not widely available in the US. Epicutaneous immunotherapy (EPIT, or the Viaskin patch) applies allergen through a skin patch and has completed Phase III trials for peanut allergy (DBV Technologies). Recombinant and modified allergen vaccines (e.g., recombinant Bet v 1 for birch pollen) aim for more consistent potency than natural extracts and are in active development. None of these have achieved FDA approval for environmental allergens as of 2025.

Allergy immunotherapy is well-established as safe for children, with the conventional minimum age being 5 years per EAACI guidelines — based on practical ability to communicate symptoms, not safety concerns. Pediatric data for SCIT are extensive: the PAT study enrolled children aged 6-14 with excellent safety profiles over 3 years. For SLIT tablets, the FDA expanded Odactra approval to children aged 5-11 in February 2025, and data from Roberts et al. (Lancet Reg Health Eur. 2024) in 1,460 children aged 5-11 found no treatment-related serious adverse events, no anaphylaxis, and no epinephrine use. A 2024 pediatric meta-analysis found SLIT had 83% fewer treatment-related adverse events than SCIT in children.