Celyad Oncology : Supplement n°1 to the Registration Document

DocuSign Envelope ID: 026DA59C-1CD3-416B-83AD-B7EBC5C0E27A

SUPPLEMENT N°1 DATED 7 SEPTEMBER 2021 TO THE REGISTRATION DOCUMENT OF 1 JUNE 2021 AND SUPPLEMENT N°1 DATED 7 SEPTEMBER 2021 TO THE PROSPECTUS OF 8

JUNE 2021

The supplement n°1 to the Registration Document (the "Supplement n°1 to the Registration Document") and the supplement n°1 to the Prospectus dated 8 June 2021 (the "Supplement n°1 to the Prospectus dated 8 June 2021") constitute supplements for the purpose of Articles 10(1) and 23(1) of the Regulation (EU) 2017/1129, as amended (the "Prospectus Regulation").

Investing in the New Shares involves a high degree of risk. An investor is exposed to the risk to lose all or part of his/her investment. Celyad is a biotech company which undertakes clinical trials that have not led to the commercialisation of any products yet and which has never been profitable. Previous positive results are no guarantee for success in subsequent studies, for regulatory approval and for market acceptance. The outbreak of the novel coronavirus (COVID-19) or any other infectious disease outbreak or other serious public health concern could result in delays to Celyad's clinical studies and could adversely affect its supply chain and work force. Investors are advised to carefully consider the information contained in the whole Prospectus and, in particular, the risks described in the section "Risk Factors". Investors must be able to bear the economic risk of an investment in shares and should be able to sustain a partial or total loss of their investment.

The English versions of both the Supplement n°1 to the Registration Document and the Supplement n°1 to the Prospectus dated 8 June 2021 were approved by the Belgian Financial Services and Markets Authority (Autorité des Services et Marchés Financiers, the "FSMA") on 7 September 2021 in its capacity as competent authority under the Prospectus Regulation. The FSMA's approval does not imply any judgment on the situation of the Company. The FSMA only approves the Supplement n°1 to the Registration Document and the Supplement n°1 to the Prospectus dated 8 June 2021 as meeting the standards of completeness, comprehensibility and consistency imposed by Prospectus Regulation. Such approval should not be considered as an endorsement of the quality of the New Shares.

The Supplement n°1 to the Registration Document and the Supplement n°1 to the Prospectus dated 8 June 2021 have been translated into French. The Company is responsible for the consistency between the French and English versions of the Supplement n°1 to the Registration Document and the Supplement to the Prospectus dated 8 June 2021. Without prejudice to the responsibility of the Company for the inconsistencies between the different language versions of the Supplement n°1 to the Registration

DocuSign Envelope ID: 026DA59C-1CD3-416B-83AD-B7EBC5C0E27A

Document and the Supplement n°1 to the Prospectus dated 8 June 2021, in the case of discrepancies between the different versions of these supplements the English version will prevail. The Supplement n°1 to the Registration Document and the Supplement n°1 to the Prospectus dated 8 June 2021will be published on the website of the Company (www.celyad.com) and will also be made available to investors, at no cost, at the Company's registered office. Following their approval, these supplements, together with a French translation, will be notified by the FSMA to the AMF in France in accordance with the European passport mechanism and pursuant to Article 25(1) of the Prospectus Regulation. This passport does not imply any judgement by the AMF on the merits or the quality of the Company or the Shares.

Save as disclosed in these supplements, there has been no other significant new factor, material, mistake or inaccuracy relating to information included in these documents since the date of publication of the Prospectus and the Registration Document.

The Board of Directors of Celyad assumes responsibility for the content of these Supplement n°1 to the Registration Document and the Supplement n°1 to the Prospectus dated 8 June 2021. The Board of Directors declares that, to the best of its knowledge, the information contained in these supplements is in accordance with the facts and that these supplements make no omission likely to affect their imports.

On behalf of the Board of Directors,

___________________

Filippo Petti Managing Director

Table of contents:

1. Supplement n°1 to the Registration Document dated 1 June 2021
2. Supplement n°1 to the Prospectus dated 8 June 2021

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DocuSign Envelope ID: 026DA59C-1CD3-416B-83AD-B7EBC5C0E27A

SUPPLEMENT N°1 DATED 7 SEPTEMBER 2021 TO THE REGISTRATION DOCUMENT OF

1 JUNE 2021

The Supplement n°1 to the Registration Document dated 7 September 2021 is supplemental to the Company's registration document as approved by the FSMA on 1st June 2021 (the "Registration Document").

In order to ensure that the information contained in the Registration Document is up-to-date, as required by the Prospectus Regulation, the Registration Document is deemed to be amended as set out below.

New information to Section 2.9.1 - Information incorporated by reference

On 4 August 2021, Celyad published its 2021 Half Year Financial Report, accompanied by a press release entitled "Celyad Oncology reports first half 2021 financial results and recent business highlights".

The aforementioned documents are incorporated by reference in the Registration Document. A copy of these documents, incorporated by reference in the Registration Document, may be obtained without charge from the Company's registered office and the Company's website (www.celyad.com).

If documents which are incorporated by reference themselves incorporate any information or other documents therein, either expressly or implicitly, such information of other documents will not form part of this Supplement n°1 to the Registration Document for the purposes of the Prospectus Regulation, except where such information or other documents are specifically incorporated by reference or attached to this Supplement n°1 to the Registration Document.

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DocuSign Envelope ID: 026DA59C-1CD3-416B-83AD-B7EBC5C0E27A

SUPPLEMENT N°1 DATED 7 SEPTEMBER 2021 TO THE PROSPECTUS

OF 8 JUNE 2021

The Supplement n°1 to the Prospectus dated 8 June 2021, which is dated 7 September 2021, is supplemental to the Prospectus, which consists of:

• the Company's registration document as approved by the FSMA on 1st June 2021 (the "Registration Document").
• the securities note as approved by the FSMA on 8 June 2021 (the "Securities Note") and the summary note as approved by the FSMA on 8 June 2021 (the "Summary Note") relating to the admission to trading of up to 2,588,977 new shares with ISIN number BE0974260896-XBRU (the "New Shares") on Euronext Brussels and Euronext Paris.

The Registration Document, the Summary Note and the Securities Note, together with this Supplement n°1 to the Prospectus dated 8 June 2021 constitute a prospectus within the meaning of Article 10 of the Prospectus Regulation (together the "Prospectus"). Terms defined in these documents shall, unless the context otherwise requires, have the same meaning when used in this Supplement n°1 to the Prospectus dated 8 June 2021.

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DocuSign Envelope ID: 026DA59C-1CD3-416B-83AD-B7EBC5C0E27A

1. NUMBER OF NEW SHARES

On 3 September 2020, the Company conditionally issued up to 2,777,777 new Shares, such issue being conditional upon the effective placement and subscription of these new shares. Of these 2,777,777 new shares, 188,800 have already been subscribed on May and June 2021 and 2,588,977 new shares are still outstanding (the "New Shares"). The placement of the New Shares will be organised under the responsibility of the board of directors of the Company. With respect to this placement of the New Shares, the Company entered into a sale agency agreement with Jefferies LLC (the "Sale Agency Agreement") pursuant to which the Company may from time to time and over a limited period of time of up to 36 months (i.e. until 2 September 2023) sell the New Shares through at-the-market ("ATM") offerings, with Jefferies acting as sales agent. The ATM offerings will be offerings to the public of American Depositary Shares ("ADS") conducted in the United States through the Nasdaq Market or any other existing U.S. trading market for the ADS. No sales will be conducted in Belgium, France or through Euronext.

The New Shares will be subscribed at the market price of the share, without any discount. The subscriptions and the effective issuance of the New Shares will be acknowledged, from time to time and over a limited period of time until 2 September 2023, by the Company through notarial deeds, as prescribed by the BCCA. Appropriate information formalities will be implemented by the Company in that respect.

At the date of this Supplement, 188,800 Shares have already been issued under the ATM (out of the 2,777,777 new Shares initially and conditionally issued) and subscribed by Jefferies LLC. Such issuances took place as follows :

1. • 182,000 Shares have been subscribed on 21st May 2021 at a price of EUR 4.58 per share, for a total proceed of EUR 833,560.
   • 6,800 Shares have been subscribed on 14 June 2021 at a price of EUR 4.98 per share, for a total proceed of EUR 33,864.
2. New information

In order to ensure that the information contained in the Prospectus is up-to-date, as required by the Prospectus Regulation, the Securities Note and the Summary are deemed to be amended as set out below.

1. Securities Note

1. Amendments to section 3.1- Capitalisation and Indebtedness

The following table sets forth the capitalisation and indebtedness of the Company as of 30th June 2021. This information presented as of 30th June 2021 should be read in conjunction with the Company's financial statements and the information in the Registration Document. There have been no material changes to Celyad Oncology's consolidated capitalisation and net financial indebtedness since 30th June except:

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