Understanding Pollen Seasons: When to Start Immunotherapy for Maximum Benefit

If you suffer from seasonal allergies, you know the drill: every spring, summer, or fall, pollen counts rise and so do your symptoms. Antihistamines and nasal sprays can manage the worst of it, but they do nothing to address the underlying immune dysfunction. Sublingual immunotherapy (SLIT) can -- but timing matters. Starting treatment at the right point in relation to your pollen season can significantly affect how well it works.

In this article, we break down the major pollen seasons, explain the difference between pre-seasonal and co-seasonal immunotherapy, and share what the clinical evidence says about optimizing your treatment timeline.

The Three Major Pollen Seasons

Pollen seasons vary by geography, but in most temperate climates -- including much of the United States -- they follow a predictable pattern:

• Tree pollen season (February - May): Trees like oak, cedar, birch, and elm release pollen earliest. In the southern U.S., mountain cedar can begin pollinating as early as December. Birch pollen is a major trigger in northern states and is closely associated with food-related allergic reactions.
• Grass pollen season (May - July): Timothy grass, Bermuda grass, ryegrass, and other species dominate late spring through mid-summer. Grass pollen is one of the most common causes of allergic rhinitis worldwide.
• Weed pollen season (August - November): Ragweed is the primary culprit, producing billions of pollen grains per plant per season. A single ragweed plant can release up to one billion pollen grains, and the pollen can travel hundreds of miles on the wind.

For polysensitized patients -- those allergic to multiple pollen types -- symptoms can span from February through November, effectively creating a year-round burden that demands more than seasonal medication management.

Pre-Seasonal vs. Co-Seasonal SLIT: What the Research Shows

One of the most clinically relevant questions in sublingual immunotherapy is when to begin treatment relative to the pollen season. Two primary approaches have been studied:

Pre-seasonal SLIT

Pre-seasonal immunotherapy involves starting treatment 2-4 months before the anticipated pollen season begins. The rationale is to allow sufficient time for immune modulation -- IgG4 blocking antibody production and regulatory T-cell activation -- before the patient encounters high allergen loads in the environment.

Wahn et al. conducted a pivotal randomized, double-blind, placebo-controlled trial evaluating pre-seasonal sublingual immunotherapy with a five-grass pollen tablet in children and adolescents. Treatment was initiated approximately 4 months before grass pollen season and continued through the season. The study demonstrated a 28% reduction in rhinoconjunctivitis symptom scores and a 34% reduction in rescue medication use compared to placebo.¹

"Pre-seasonal initiation of sublingual immunotherapy with grass pollen tablets provided clinically significant symptom relief during the subsequent pollen season, with benefits increasing over successive treatment years." -- Wahn et al., Journal of Allergy and Clinical Immunology, 2009

Co-seasonal SLIT

Co-seasonal treatment begins during the pollen season itself. While this approach provides less lead time for immune modulation, several studies have demonstrated that co-seasonal SLIT can still provide meaningful benefit, particularly when continued across multiple seasons.

The key advantage of co-seasonal initiation is practical: patients who miss the pre-seasonal window do not need to wait an entire year to begin treatment. The evidence suggests that starting during the season is better than not starting at all.

The Landmark Grass Pollen Tablet Trials

The strongest timing evidence comes from the large-scale grass pollen sublingual tablet trials. Didier et al. conducted a major European randomized controlled trial evaluating a 300IR grass pollen sublingual tablet (Oralair) initiated either 2 months or 4 months before the grass pollen season. Both regimens were continued through the season.²

Key findings from the Didier trial:

• Both the 2-month and 4-month pre-seasonal start groups showed significant symptom improvement over placebo
• The 4-month pre-seasonal group showed a trend toward greater benefit, though the difference between the two active groups was not statistically significant
• Symptom improvement was most pronounced during peak pollen exposure periods
• Benefits accumulated over successive treatment years, with the greatest improvement seen in the second and third seasons

These results suggest that while earlier initiation may provide a modest additional benefit, the most important factor is consistent daily dosing over multiple seasons.

Continuous vs. Discontinuous Treatment: The Durham Evidence

A critical question for patients and clinicians is whether SLIT should be taken continuously (year-round) or only during defined treatment periods. Durham et al. addressed this in a landmark study examining the long-term efficacy and disease-modifying potential of grass pollen SLIT.³

The Durham study followed patients through three years of continuous daily grass pollen SLIT and then monitored outcomes for two additional years after treatment cessation. The results were compelling:

• Continuous daily treatment for three years produced sustained symptom improvement that persisted for at least two years after stopping
• This "carry-over" effect suggests genuine disease modification rather than mere symptom suppression
• Patients who received continuous treatment showed greater and more durable benefit compared to those on discontinuous regimens

"Three years of continuous sublingual immunotherapy with grass pollen tablets induced sustained clinical benefit that persisted for at least two years following treatment discontinuation, consistent with disease modification." -- Durham et al., Journal of Allergy and Clinical Immunology, 2012

This evidence forms the basis for current recommendations that SLIT should be continued for a minimum of three years to achieve lasting disease modification.

Practical Timing Recommendations

Based on the cumulative evidence, here are evidence-based timing recommendations for patients considering sublingual immunotherapy for pollen allergies:

• Ideal start: 3-4 months before your primary pollen season begins. For spring tree pollen allergies, this means starting in November or December. For grass pollen, February or March. For ragweed, April or May.
• Acceptable start: 2 months before season onset. The Didier trial showed meaningful benefit even with this shorter lead time.
• Still beneficial: Co-seasonal start. While not optimal, beginning during the season provides a foundation for improved outcomes in subsequent seasons.
• Duration: Plan for at least 3 years of continuous daily treatment to achieve disease modification and lasting benefit.
• Multi-allergen patients: If you are sensitized to pollens across multiple seasons, year-round continuous SLIT is the most practical and effective approach.

At OLLEREG, our Pollen Relief Spray is designed for continuous daily use, allowing patients to maintain consistent allergen exposure regardless of season. This year-round approach aligns with the strongest evidence for long-term efficacy and disease modification.