This presentation contains "forward-looking statements" within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, each as amended. These statements may be ide

Top-line Results for the Phase 2

VALIANT Trial in Severe Asthma February 11, 2026 2026 Upstream Bio, Inc. Exhibit 99.1

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clinical development of verekitug for the treatment of severe asthma, CRSwNP and COPD, including the timing, progress and results of ongoing and planned clinical trials; expectations for future discussions with regulatory authorities and the

potential of the endpoints of our clinical trials to produce data that could support submissions for product approval; our expectations regarding the differentiation, safety, efficacy, tolerability, or extended dosing interval of verekitug;

expectations for the size and growth potential of the market for verekitug and our ability to serve that market; additional potential indications for verekitug; and our expected cash runway.. Forward-looking statements are based on our current

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clinical development; the results of preclinical studies, or clinical studies not being predictive of future results in connection with future studies; the initiation, timing, progress and results of clinical trials; our ability to fund our

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Agenda Introduction Rand Sutherland,

MD Chief Executive Officer VALIANT Phase 2 Top-line Results Aaron Deykin, MD Chief Medical Officer and Head of R&D Path Forward Rand Sutherland, MD Analyst Q&A Rand Sutherland, MD Aaron Deykin, MD 2026 Upstream Bio, Inc.

Phase 2 VALIANT study met primary

endpoint in reduction of AAER 2026 Upstream Bio, Inc. AAER, annualized asthma exacerbation rates * Placebo-corrected 400mg q24w 39% reduction in AAER (p<0.02) 139mL improvement in FEV1 26.3ppb* reduction in FeNO 44.9% * reduction vs

baseline Verekitug was generally well tolerated, with a safety profile consistent with prior studies Statistically significant and clinically meaningful reductions in AAER with verekitug dosed for up to 60 weeks at

100mg every 12 weeks and 400mg every 24 weeks 100mg q12w 56% reduction in AAER (p<0.0003) 122mL improvement in FEV1 20.4ppb* reduction in FeNO 43.5% * reduction vs baseline Both verekitug doses also delivered clinically meaningful

improvements in lung function (FEV1) and exhaled nitric oxide (FeNO)

About Upstream Bio Clinical-stage

immunology company focused on severe respiratory diseases 2026 Upstream Bio, Inc. Developing the only known clinical-stage antagonist of the TSLP receptor Verekitug's pharmacology is unique and characterized by rapid, complete and

sustained occupancy of the TSLP receptor for up to 24 weeks after the last dose TSLP, thymic stromal lymphopoietin; CRSwNP, chronic rhinosinusitis with nasal polyps; COPD, chronic obstructive pulmonary disease. 1 Preliminary, unaudited and subject

to change. Studying verekitug across multiple indications with high unmet need All trials randomized and placebo-controlled, with registration-enabling endpoints VALIANT: Ph 2 trial in severe asthma - Reporting out positive top-line

results today VALOUR, long-term extension study - Currently enrolling eligible participants who completed VALIANT VIBRANT: Ph 2 trial in CRSwNP - Reported positive top-line results in September 2025 VENTURE: Ph 2 trial in COPD -

Currently enrolling and enrollment >60% complete Addressing significant commercial opportunities Severe asthma and COPD markets are expanding and expected to drive a $35B+ global biologics market by 2033 Cash, cash equivalents and short-term

investments of ~$341.5M as of Dec 31, 20251 expected to fund planned operations through 2027

Large and growing commercial

opportunity in core indications, with asthma and COPD alone expected to be a $35B+ global biologics market in 2033 Biologic eligible severe asthma patients in the US1 of biologic sales are in the US2 Severe Asthma 2023: $7.5B2 2032e: $12.6B2 CRSwNP

2025: >$1.5B5,6* COPD 2033e: $23B2 Biologics to treat CRSwNP have been available for only ~6 years, now with 4 approved treatment options Use of biologics is growing rapidly, with >20% increases in number of claims, prescribing HCPs, and

patients receiving biologics to manage CRSwNP4 Current global biologics sales in CRSwNP alone estimated to be $1.5B+ annually5,6 ~300K+ biologic eligible CRSwNP patients in the US1, with significant potential for growth as the proportion of eligible

patients taking a biologic increases and novel biologics enter the market Currently ~1.1M severe COPD patients in the US, expected to be ~3.5M globally by 20332; ~70% of 2033 biologic sales are expected to be in the US2 Currently only 2 biologics

are approved for the treatment of COPD If approved in this indication, Tezspire is projected to reach global annual sales of over $5B for COPD alone in 20332 1 Upstream Bio Analysis 2025 2 Datamonitor 3 IQVIA Claims Data (Unique Asthma Pts on

Biologics FY '24, Pt Growth Rate 20-'24) 4 Amgen Investor Presentation May 2024 5 Symphony Claims Data (June '24 - May '25) 6 Manufacturer Sales and Pricing Disclosures 2026 Upstream Bio, Inc. of eligible

patients with severe asthma are currently estimated to receive biologic therapies3 In 2033, peak global sales for all approved biologics in severe asthma are estimated to be at least $12.5 billion2 Tezspire is projected to reach peak global annual

sales of over $3B for severe asthma alone in 2032,2 and achieved more than 20% of new to brand share of prescriptions in the US in its first commercial year4 ~1.3M ~80% <25% >$12.5B $3B

Phase 2 VALIANT Trial in Severe Asthma

Aaron Deykin, MD Chief Medical Officer and Head of R&D 2026 Upstream Bio, Inc.

VALIANT phase 2 trial design Enrolled

478 patients with severe asthma across 15 countries1-3 Global, randomized, placebo-controlled phase 2 trial with up to 60-week treatment period (NCT06196879) 2026 Upstream Bio, Inc. Follow-up Verekitug 400 mg SC q24w (Medium dose) Placebo SC

q12w 1:1:1:1 N = 478 randomized 0 12 24 36 48 60 64

Week Screening Verekitug 100 mg SC q12w (High dose) Verekitug 100 mg SC q24w (Low dose) ACQ-6, Asthma Control Questionnaire; BD, bronchodilator; FeNO, fractional exhaled nitric oxide; FEV1, forced expiratory volume in 1 second; ICS,

inhaled corticosteroid; q w, every weeks; SC, subcutaneous; SCS, systemic corticosteroids. 1. Data on file. Table 14.1.1.1. 2. NCT06196879. https://clinicaltrials.gov/study/NCT06196879. 3. VALIANT Study Protocol V4.0. Key inclusion

criteria: Aged 18-80 years Physician-diagnosed asthma for at least 12 months Pre-BD FEV1 30% and 80% predicted with evidence of BD reversibility Treatment with medium/high dose ICS 3 months Documented history of asthma

exacerbation(s) in past 12 months as defined by any of 2 events req. SCS 1 event req. inpatient care 24 hrs 1 event req. SCS + FeNO 50 ppb ACQ-6 1.5 Variable Treatment Period Design: Min 24 weeks - Max

60 weeks Primary endpoint: Annualized Asthma Exacerbation Rate (AAER) from baseline up to Week 60 90% power to detect 50% reduction vs placebo Secondary endpoints: (Study not powered for secondary endpoints) Change from baseline to week 60

Pre-BD FEV1 FeNO ACQ-6 Pre-specified 24-week estimate for all secondary endpoints

VALIANT ITT population disposition 433

(91%) of participants completed treatment 2026 Upstream Bio, Inc. *Including one participant who was not dosed due to AE. AE, adverse event; ITT, intent-to-treat; q w, every weeks. Data on file. Table 14.1.1.1. 478

participants randomized Placebo n=119 Verekitug 400 mg q24w n=118 Completed treatment n=110 (93%) Completed treatment n=105 (88%) Treatment discontinuation n=14 Adverse event: 2 Participant/guardian decision: 7 Lost to follow-up: 3 Protocol

Deviation: 2 Received placebo n=119 Received verekitug n=118 Verekitug 100 mg q12w n=121 Verekitug 100 mg Q24W n=120 Completed treatment n=109 (91%) Received verekitug n=119 Completed treatment n=109 (90%) Received verekitug n=121 Treatment

discontinuation n=11 Adverse event: 3* Participant/guardian decision: 5 Lost to follow-up: 1 Other: 2 Treatment discontinuation n=8 Adverse event: 1 Participant/guardian decision: 6 Lost to follow-up: 1 Treatment discontinuation n=12 Adverse event:

4 Participant/guardian decision: 3 Lost to follow-up: 3 Other: 2

Baseline characteristics were

balanced across treatment groups Reflect an uncontrolled, severe asthma population Verekitug (n=) Age, mean (SD), years1 54.6 (12.15) 53.8 (11.74) 51.7 (15.37) 54.0 (13.06) Female sex, n (%)1 73 (60.3) 79 (66.9) 72 (60.0) 81 (68.1) Race, n (%)1

White Black/African American Other 91 (75.2) 13 (10.7) 17 (14.1) 90 (76.3) 12 (10.2) 16 (13.6) 90 (75.0) 15 (12.5) 15 (12.5) 93 (78.2) 8 (6.7) 18 (15.1) Region, n (%)1 North America Western Europe Central/Eastern Europe Rest of the world 32

(26.4) 17 (14.0) 35 (28.9) 37 (30.6) 29 (24.6) 13 (11.0) 38 (32.2) 38 (32.2) 33 (27.5) 15 (12.5) 41 (34.2) 31 (25.8) 38 (31.9) 19 (16.0) 33 (27.7) 29 (24.4) Asthma duration, mean (SD), years2 27.2 (16.98) 22.0 (17.03) 24.2 (15.28) 26.8 (17.55) ICS

use, n (%)2 Medium / high dose* 63 (52.1) / 58 (47.9) 62 (52.5) / 56 (47.5) 59 (49.2) / 61 (50.8) 61 (51.3) / 58 (48.7) Prebronchodilator FEV1, liter, mean (SD)2 1.69 (0.58) 1.75 (0.58) 1.84 (0.60) 1.77 (0.59) Baseline FeNO, mean ppb (SD)2 41.9

(44.79) 39.3 (46.47) 32.7 (28.80) 37.9 (37.20) Baseline ACQ-6 score, mean3 2.72 (0.655) 2.54 (0.573) 2.67 (0.674) 2.65 (0.674) Blood eosinophil count, cells/ L2 Median (min-max) 290 (30, 1850) 270 (30, 3090) 260 (30, 2720) 250 (30, 2090)

2026 Upstream Bio, Inc. *Median dose inhaled corticosteroids without oral corticosteroid; high dose ICS and/or oral corticosteroids; Includes Latin America, Asia-Pacific and South Africa. FeNO, fractionated exhaled nitric oxide; FEV1, forced

expiratory volume in 1 second; ICS, inhaled corticosteroid; q w, every weeks; SD, standard deviation. 1. Data on file. Table 14.1.3.1. 2. Data on file. Table 14.1.3.2.1. 3. Data on file. Table 14.2.2.4.2. Verekitug 100 mg q24w n=120

Placebo n=119 Verekitug 400 mg q24w n=118 Verekitug 100 mg q12w n=121

AE category, % of participants1 Any

TEAE Any grade 3-5 TEAEs Any TEAEs with outcome of death Any serious TEAEs Any TEAEs leading to study drug discontinuation/interruption Most common TEAE, any grade ( 5% in any cohort)2 Nasopharyngitis Bronchitis Headache Urinary tract

infection Back pain Influenza Asthma Upper respiratory tract infection Immunogenicity3 ADA positive* Verekitug was generally well tolerated across treatment groups 2026 Upstream Bio, Inc. Overall incidence of TEAEs was similar across

treatment groups Serious TEAEs were similar across treatment groups ADAs did not impact safety 62.0 6.6 0 4.1 3.3 58.5 11.0 0 6.8 0.8 62.2 8.4 0 5.0 3.4 65.5 8.4 0 8.4 1.7 *Numbers are treatment-induced and treatment-boosted ADA. Safety follow-up is

ongoing. ADA, anti-drug antibody; AE, adverse event; q w, every weeks; TEAE, treatment-emergent adverse event. 1. Data on File. Table 14.3.1.1. 2. Data on File. Table 14.3.1.2. 3. Data on File. Table 14.2.3.16.3. 5.8 5.9 5.0 5.0 3.3 2.5

4.2 6.7 10.7 7.6 7.6 10.1 7.4 6.8 1.7 2.5 5.0 2.5 1.7 3.4 6.6 2.5 6.7 5.0 2.5 4.2 5.0 5.9 2.5 2.5 5.0 5.9 60.3 50.8 60.5 -- Verekitug 100 mg q12w N=121 Verekitug 400 mg q24w N=118 Verekitug 100 mg q24w N=119 Placebo N=119

Secondary endpoints AAER

over 60 weeks (95% CI)1* 0.66 (0.47, 0.94) 0.92 (0.67, 1.27) 0.78 (0.55, 1.08) 1.52 (1.13, 2.03) Rate ratio vs placebo (95% CI) P value 0.44 (0.28, 0.69) 0.0003 0.61 (0.40, 0.93) 0.0227 0.51 (0.33, 0.79) 0.0028 - Prebronchodilator FEV1 (mL)

change from baseline at 60 weeks, LSM (95% CI)2 265 (115, 415) 281 (128, 434) 161 (5, 317) 143 (-9, 294) LSM difference vs placebo (95% CI) Nominal P value 122 (-90, 335) 0.2589 139 (-76, 353) 0.2047 18 (-198, 235) 0.8678 - FeNO (ppb)

change from baseline at 60 weeks, LSM (95% CI)3 -17.4 (-25.2, -9.6) -23.3 (-31.4, -15.1) -13.9 (-22.2, -5.6) 3.1 (-4.8, 11.0) LSM difference vs placebo (95% CI) Nominal P value -20.4 (-31.5, -9.4) 0.0003 -26.3 (-37.6, -15.0) <0.0001 -17.0 (-28.4,

-5.6) 0.0036 - ACQ-6 change from baseline at 60 weeks, LSM (95% CI)4 -1.04 (-1.39, -0.70) -1.32 (-1.67, -0.96) -1.34 (-1.71, -0.97) -1.10 (-1.45, -0.76) LSM difference vs placebo (95% CI) Nominal P value 0.06 (-0.42, 0.55) 0.8000 -0.21

(-0.70, 0.28) 0.3928 -0.24 (-0.74, 0.26) 0.3541 - Verekitug led to statistically significant improvements in AAER at 60 weeks with both q12w and q24w regimens 2026 Upstream Bio, Inc. *AAER, Rate Ratio, 95% confidence intervals, and

p-values are from a negative binomial regression model with number of asthma exacerbations as the dependent variable and fixed effects for study treatment, region, baseline steroid use as randomized, and baseline eosinophil level as randomized.

Secondary endpoints were not powered for statistical significance. ACQ-6, Asthma Control Questionnaire; AAER, annual asthma exacerbation rate; FeNO, fractional exhaled nitric oxide; FEV1, forced expiratory volume in 1 second; LSM, least

squares mean; ppb, parts per billion; q w, every weeks 1. Data on File. Table 14.2.1.1.1. 2. Data on File. Table 14.2.2.2.1.3. 3. Data on File. Table 14.2.2.3.2. 4. Data on File. Table 14.2.2.4.2. Primary endpoint Verekitug 100 mg q24w

n=120 Placebo n=119 Verekitug 400 mg q24w n=118 Verekitug 100 mg q12w n=121

2026 Upstream Bio, Inc.

Verekitug 100 mg q12w and 400 mg q24w doses led to numerical improvements in lung function and FeNO as early as week 2 and sustained over 60 weeks No. of participants Secondary endpoints were not powered for statistical significance. FeNO,

fractionated exhaled nitric oxide; FEV1, forced expiratory volume in 1 second; LSM, least squares mean; ppb, parts per billion; q w, every weeks. 1. Data on file. Table 14.2.2.2.1.3. 2. Data on file. Table 14.2.2.3.5. Verekitug 100 mg

q12w 121 118 117 114 108 115 113 78 69 64 47 40 38 23 19 Verekitug 400 mg q24w 118 115 115 112 109 112 109 82 66 60 46 38 34 21 18 Verekitug 100 mg q24w 119 115 115 113 113 113 111 84 66 62 42 35 31 21 17 Placebo 119 109 110 113 110 108 108 83 67 60