Third Quarter 2021 Financial Results Conference Call Presentation

Q3 Clinical and Financial Update

November 11, 2021

June 2020

DISCLAIMERS

Forward-Looking Statements

This presentation contains "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act of 1995. Forward-looking statements are neither historical facts nor assurances of future performance. Instead, they are based on our current beliefs, expectations and assumptions regarding the future of our business, future plans and strategies, our clinical results and other future conditions. All statements other than statements of historical facts contained in this presentation are forward-looking statements, including statements by the Company regarding future results of operations and financial position, including our anticipated cash runway; business strategy; current and prospective product candidates; anticipated milestone events; potential benefits of our product candidates and market opportunity; planned clinical trials, including, without limitation, anticipated initiation, enrollment, regulatory submission and data readout timelines; preclinical activities; product approvals; manufacturing availability; degree of market acceptance of approved products; research and development costs; current and prospective collaborations, including our collaboration with Roche and potential milestones thereunder; and prospects and opportunities for investors. In some cases, you can identify forward-looking statements by terms such as "may," "will," "should," "expects," "plans," "anticipates," "could," "intends," "targets," "projects," "contemplates," "believes," "estimates," "predicts," "potential" or "continue" or the negative of these terms or other similar expressions.

The information in this presentation, including without limitation the forward-looking statements contained herein, represent our views as of the date of this presentation. These statements are neither promises nor guarantees, but involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements. Risks and uncertainties that may cause actual results to differ materially include uncertainties inherent in the drug development process and the regulatory approval process, in particular for AT-527, our reliance on third parties over which we may not always have full control, competition for vaccines and other treatments for COVID-19, risks related to the COVID-19 pandemic on our business, and other important risks and uncertainties that are described in our most recent Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission ("SEC") and our other filings with the SEC. New risk factors and uncertainties may emerge from time to time, and it is not possible to predict all risk factors and uncertainties. Accordingly, readers are cautioned not to place undue reliance on these forward-looking statements.

Except as required by applicable law, we do not plan to publicly update or revise any forward-looking statements contained herein, whether as a result of any new information, future events, changed circumstances or otherwise. No representations or warranties (expressed or implied) are made about the accuracy of any such forward-looking statements.

Industry Information

Market data and industry information used throughout this presentation are based on management's knowledge of the industry and the good faith estimates of management. We also relied, to the extent available, upon management's review of independent industry surveys and publications and other publicly available information prepared by a number of third- party sources. All of the market data and industry information used in this presentation involves a number of assumptions and limitations, and you are cautioned not to give undue weight to such estimates. Although we believe that these sources are reliable, we cannot guarantee the accuracy or completeness of this information, and we have not independently verified this information. While we believe the estimated market position, market opportunity and market size information included in this presentation are generally reliable, such information, which is derived in part from management's estimates and beliefs, is inherently uncertain and imprecise. No representations or warranties are made by the Company or any of its affiliates as to the accuracy of any such statements or projections. Projections, assumptions and estimates of our future performance and the future performance of the industry in which we operate are necessarily subject to a high degree of uncertainty and risk due to a variety of factors, including those described above. These and other factors could cause results to differ materially from those expressed in our estimates and beliefs and in the estimates prepared by independent parties.

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AT-527 AT-527 Addresses Key Challenges of COVID-19:

Oral Pill with MOA Designed to Inhibit Viral Replication

• Oral direct-acting antivirals (DAAs) are complementary to vaccines, easy to access with a prescription
• Targets viral RNA polymerase, a highly conserved enzyme critical to viral replication
• Unique differentiated mechanism with dual targets:

• Chain termination (RdRp) without introducing mutations in the virus

• NiRAN inhibition	N -

AT-527

Nsp12 Functional Domains

SARS-Cov-2

NiRAN		RdRp	- C

• Potentially creating a high barrier to resistance with broad antiviral coverage to	51	249		366		932
coronaviruses and different variants of SARS-CoV-2		RdRp	=	R	d	R	p
			NA- ependent		NA olymerase
• Non-mutagenic in mammalian cells in vitro, no effect on reproductive toxicology	NiRAN = Nidovirus RdRp-Associated
	Nucleotidyltransferase
and non-teratogenic; no changes to the SARS-CoV-2 genome

• Minimal drug-druginteraction, AT-527 is a weak inhibitor of CYP3A & no dose adjustment expected for co-administration of drugs that are CYP3A substrates
• Global collaboration with Roche with multiple clinical trials advancing in parallel, including global Phase 3 MORNINGSKY trial

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COVID-19 COVID-19 Continues to Evolve with New Variants and Viral Kinetics

• Almost 6,000 variants have been sequenced

• New variants associated with increased transmissibility, neutralization resistance and disease severity

• Delta Plus (AY.4.2) is a subvariant of the Delta variant

• Mutation associated with increased risk of transmission and reinfection

• Leading to new COVID-19 surge in Europe and other areas worldwide

• https://doi.org/10.1101/2021.08.20.21262342doi: medRxiv preprint

AT-527

In Vitro Activity Against COVID-19 Variants of Concern and/or of Interest

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