Preliminary Proxy Statement

Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

Definitive Proxy Statement

Definitive Additional Materials

Soliciting Material under §240.14a-12

No fee required.

Fee paid previously with preliminary materials.

Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11.

What does this transaction mean for me?

This transaction recognizes the power and future additional potential of our pioneering ADC technology, as Seagen becomes central to Pfizer’s oncology business.

Together with Pfizer, we will create a leading oncology company capable of more quickly delivering life-changing cancer therapies to more patients worldwide.

Pfizer values the role of the Seagen team in advancing our shared mission.

This combination will create opportunities for many employees.

Pfizer shares our patient-centric culture and recognizes that the deep and differentiated expertise of the Seagen team will be critical as we look to advance our joint mission.

How will my work change?

For right now, nothing changes.

Until the transaction closes, Seagen will continue to operate as a separate and independent company. We will remain focused on delivering on our core business goals and continuing to develop and commercialize cancer medicines for patients.

A small executive integration planning team will be established to coordinate planning activities with Pfizer and manage the transaction through regulatory clearance and close. Updates on integration planning will be provided as we progress.

Will my role or compensation change?

There is no change to your role now. Until the transaction closes, Seagen will continue to operate as a separate and independent company.

Seagen’s compensation packages and benefits will generally remain unchanged through the close. We will continue to make merit adjustments, bonus payouts, and equity grants. Annual August equity grants are expected to continue and be delivered in full value restricted stock units (RSUs).

Employees in good standing will receive a retention award. This will be explained to employees in coming weeks.

Pfizer has strong and comprehensive compensation and benefits programs. Details will be shared as we get closer to deal close.

How will this impact my stock options and RSUs?

Unvested stock options and RSUs that you currently hold will accelerate and vest on the day the deal closes at the purchase price.

Will there be any other retention incentive provided to Seagen employees (in addition to equity accelerating and vesting at close)?

Retention awards will be provided to Seagen employees in good standing to secure performance and employment through the close.

Seagen employees will receive a letter at the beginning of April.

The Executive Committee (EC) is working together to ensure equitable consideration across the entire organization.

When will the transaction close? What are the expected key milestones leading up to closing?

The transaction is expected to close in late 2023 or early 2024.

We are committed to providing updates and being as transparent as possible as the transaction progresses. Please note that certain legal and financial considerations will restrict our ability to disclose certain details.

We will share relevant information and updates on a dedicated Transition Updates section of The Linker.

What role is Seagen’s management team expected to play in the combined company?

Seagen’s management team has a commitment to deliver on our 2023 goals and ensure a positive transition for our employees.

Longer term decisions have not yet been discussed.

Will Pfizer retain Seagen’s offices?

For now, nothing immediately changes and Seagen and Pfizer will continue to operate independently, as we work to satisfy customary closing conditions, including regulatory approvals.

Pfizer will maintain Seagen sites in the Seattle-area and South San Francisco, review other sites and retain most of Seagen’s employees to continue our important work.

We will provide updates periodically throughout the process.

What if I am not offered a job with Pfizer?

While we expect most Seagen employees to have opportunities going forward, if your job is not retained due to this acquisition, you will be provided a severance package.

In the coming weeks and months, we will work to finalize our plans to bring our companies together. We will provide updates periodically throughout the process.

Can I reach out to people I know at Pfizer?

No, until close, Pfizer and Seagen will continue to operate independently.

Seagen employees should not reach out to Pfizer counterparts unless authorized by leadership to do so.

Continue to maintain our information and intellectual property as you do today.

Can I respond to any media or investor inquiries?

No, all media inquiries must be directed to CorporateCommunications@Seagen.com and any investor inquiries must be directed to InvestorRelations@Seagen.com.

Can I discuss this transaction on social media?

No. Regulators have strict rules around employees communicating about a potential transaction. You should not discuss or respond to discussions on social media regarding the merger in any manner.

How will this impact our in-office work policies?

Continue to work as you have been with our current workplace guidance.

Our corporate goals, department goals, individual goals and governance processes will be largely unchanged during the transition period.

Who should I contact if I have other questions? When will I hear more information?

Please share any questions or concerns with your Team Leader or HR Partner. Additional information will be shared over time, and we will not have all the answers right away. We will work diligently to get you information as it becomes available.

In the coming weeks, we plan to have a dedicated section for Transition Updates on The Linker to provide you with the most up-to-date information about the transaction and key milestones.