The purpose of this study is to establish the efficacy, safety and tolerability of barzolvolimab in adult participants with Chronic Spontaneous Urticaria (CSU) inadequately controlled by non-sedating second generation H1-antihistamines in comparison to placebo.

This is a global, multicenter, randomized, double-blind, parallel group, placebo-controlled phase 3 study investigating the efficacy, safety and tolerability of barzolvolimab in adult participants with Chronic Spontaneous Urticaria (CSU) who are symptomatic despite treatment with non-sedating second generation H1-antihistamines at 1-4 times the locally approved dose.

There is a screening period of up to 4 weeks, followed by a 24-week placebo-controlled treatment period, a 28-week active treatment period where all participants receive barzolvolimab followed by a 16-week treatment free period.

Approximately 915 adult participants (610 in the active arms and 305 in the placebo arm) will be randomly assigned to the treatment arms.

Key Inclusion Criteria:

1. Males and females, >/= 18 years of age.

2. Chronic spontaneous urticaria (CSU) >/= 6 months prior to Screening (Visit 1).

3. CSU despite the use of a stable regimen of second generation non-sedating H1-antihistamine as defined by:

   1. The presence of hives for >/= 6 weeks at any time prior to Visit 1 despite the use of non-sedating H1-antihistamines.
   2. Must be on a stable regimen of second generation non-sedating H1-antihistamine for >/= 4 weeks prior to study treatment.
   3. UAS7 of >/= 16 and ISS7 of >/= 8 during the 7 days prior to study treatment.

4. Normal blood counts and liver function tests.

5. Both males and females of child-bearing potential must agree to use highly effective contraceptives during the study and for 150 days after treatment.

6. Willing and able to complete a daily symptom electronic diary and comply with study visits.

7. Participants with and without prior biologic experience are eligible.

Key Exclusion Criteria:

1. Women who are pregnant or nursing.
2. Chronic urticaria whose predominant manifestation is due to CIndU.
3. Other diseases associated with urticaria.
4. Active pruritic skin condition in addition to CSU.
5. Medical condition that would cause additional risk or interfere with study procedures.
6. Known HIV, hepatitis B or hepatitis C infection.
7. Vaccination of a live vaccine within 30 days prior to Screening (Visit 1) (subjects must agree to avoid live vaccinations during the study). Inactivated vaccines are allowed such as seasonal influenza injection or authorized COVID-19 vaccine.
8. History of anaphylaxis.
9. Prior treatment with barzolvolimab.

There are additional criteria that your study doctor will review with you to confirm you are eligible for the study.