Background: Evidence for the efficacy of combined PD-1 and HER2 blockade with chemotherapy on progression-free and overall survival in HER2-positive gastro-oesophageal cancer is scarce. The first interim analysis of the randomised, phase 3 KEYNOTE-811 study showed a superior objective response with pembrolizumab compared with placebo when added to trastuzumab plus fluoropyrimidine and platinum-based chemotherapy. Here, we report results from protocol-specified subsequent interim analyses of KEYNOTE-811., Methods: The randomised, phase 3 KEYNOTE-811 trial involved 168 medical centres in 20 countries worldwide. Patients aged 18 years or older with locally advanced or metastatic HER2-positive gastro-oesophageal junction adenocarcinoma, without previous first-line treatment, were randomly assigned (1:1) by an integrated interactive voice-response and web-response system to intravenous pembrolizumab 200 mg or placebo, both to be combined with standard chemotherapy (fluoropyrimidine and platinum-based therapy) plus trastuzumab every 3 weeks for up to 35 cycles or until disease progression, unacceptable toxic effects, or investigator or participant-initiated withdrawal. Randomisation used a block size of four and was stratified by region, PD-L1 status, and chemotherapy. Dual primary endpoints were progression-free and overall survival, analysed by intention to treat. Safety was assessed in all randomly assigned patients who received at least one dose of study treatment according to the treatment received. KEYNOTE-811 is registered with ClinicalTrials.gov (NCT03615326) and is active but not recruiting., Findings: Between Oct 5, 2018, and Aug 6, 2021, 698 patients were assigned to pembrolizumab (n=350) or placebo (n=348). 564 (81%) were male and 134 (19%) were female. At the third interim analysis, 286 (82%) of 350 patients in the pembrolizumab group and 304 (88%) of 346 in the placebo group who received treatment had discontinued treatment, mostly due to disease progression. At the second interim analysis (median follow-up 28·3 months [IQR 19·4-34·3] in the pembrolizumab group and 28·5 months [20·1-34·3] in the placebo group), median progression-free survival was 10·0 months (95% CI 8·6-11·7) in the pembrolizumab group versus 8·1 months (7·0-8·5) in the placebo group (hazard ratio [HR] 0·72, 95% CI 0·60-0·87; p=0·0002). Median overall survival was 20·0 months (17·8-23·2) versus 16·9 months (15·0-19·8; HR 0·87 [0·72-1·06]; p=0·084). At the third interim analysis (median follow-up 38·4 months [IQR 29·5-44·4] in the pembrolizumab group and 38·6 months [30·2-44·4] in the placebo group), median progression-free survival was 10·0 months (8·6-12·2) versus 8·1 months (7·1-8·6; HR 0·73 [0·61-0·87]), and median overall survival was 20·0 months (17·8-22·1) versus 16·8 months (15·0-18·7; HR 0·84 [0·70-1·01]), but did not meet prespecified criteria for significance and will continue to final analysis. Grade 3 or worse treatment-related adverse events occurred in 204 (58%) of 350 patients in the pembrolizumab group versus 176 (51%) of 346 patients in the placebo group. Treatment-related adverse events that led to death occurred in four (1%) patients in the pembrolizumab group and three (1%) in the placebo group. The most common treatment-related adverse events of any grade were diarrhoea (165 [47%] in the pembrolizumab group vs 145 [42%] in the placebo group), nausea (154 [44%] vs 152 [44%]), and anaemia (109 [31%] vs 113 [33%])., Interpretation: Compared with placebo, pembrolizumab significantly improved progression-free survival when combined with first-line trastuzumab and chemotherapy for metastatic HER2-positive gastro-oesophageal cancer, specifically in patients with tumours with a PD-L1 combined positive score of 1 or more. Overall survival follow-up is ongoing and will be reported at the final analysis., Funding: Merck Sharp & Dohme., Competing Interests: Declaration of interests YYJ reports research funding to their institution from MSD, the US National Cancer Institute, the US Department of Defense, Cycle of Survival, Fred's Team, Rgenix, Bayer, Genentech/Roche, Bristol-Myers Squibb, and Eli Lilly; fees for advisory board participation from Inspira, Merck Serono, Bristol-Myers Squibb, Eli Lilly, Pfizer, Bayer, Imugene, Merck Sharp & Dohme, Daiichi Sankyo, Zymeworks, SeaGen, Basilea Pharmaceutical, and AstraZeneca; and equity in Inspira. AK reports research funding to their institution from AstraZeneca, Ono Pharmaceutical, Daiichi Sankyo, and MSD; consulting fees from Zymeworks, MSD, and Astellas Pharma; and honoraria from Bristol-Myers Squibb, Daiichi Sankyo, Ono Pharmaceutical, Eli Lilly, and Taiho Pharmaceutical. YB, JX, PY, LSW, YO, MB, HCC, and S-KQ report research funding to their institutions from Merck Sharp & Dohme, a subsidiary of Merck & Co (MSD). SL reports research funding to their institution from Amgen, Astellas, AstraZeneca, Bayer, Bristol-Myers Squibb, Daichii Sankyo, Hutchinson, Incyte, Merck Serono, Mirati, MSD, Pfizer, Roche, and Servier; consulting fees from Amgen, AstraZeneca, Bristol-Myers Squibb, Daichii Sankyo, Incyte, Lilly, Merck Serono, MSD, Servier, and Astellas; and honoraria from Amgen, Bristol-Myers Squibb, Incyte, GSK, Lilly, Merck Serono, MSD, Pierre Fabre, Roche, and Servier. JPM reports grants from MSD, Bristol-Myers Squibb, Zymeworks, Astellas, Bayer, and Diachii; honoraria from MSD, Astellas, Bayer, and Bristol-Myers Squibb; and reimbursement for travel from MSD, Daichii, and BMS. LS reports research funding to their institution from MSD, Beijing Xiantong Biomedical Technology, Qiku Pharmaceutical Co, Zaiding Pharmaceutical (Shanghai) Co, Jacobio Pharmaceuticals Co, Beihai Kangcheng (Beijing) Medical Technology Co, Boehringer Ingelheim, Harbour, and Merck; and consulting fees from Harbour and Merck. KS reports research funding to their institution from Astellas Pharma, Ono Pharmaceutical, Daiichi Sankyo, Taiho Pharmaceutical, Chugai Pharma, MSD, Amgen, and Eisai; consulting fees from Eli Lilly and Company, Bristol-Myers Squibb, Takeda Pharmaceutical, Novatis, AbbVie, Daiichi Sankyo, Taiho Pharmaceutical, GSK, Amgen, Boehringer Ingelheim, MSD, Astellas, Guardant Health Japan, and Janssen; and honoraria from Bristol-Myers Squibb, Takeda Pharmaceuticals, and Janssen. EVC reports research funding to their institution from Bayer, Boehringer Ingelheim, Celgene, Ipsen, Lilly, Merck Sharp & Dohme, Merck KGaA, Novartis, Rocher, and Servier; and fees for advisory board consulting for Array, AstraZeneca, Bayer, Biocartis, Bristol-Myers Squibb, Celgene, Daiichi, Halozyme, GSK, Incyte, Ipsen, Lilly, Merck Sharp & Dohme, Merck KGaA, Novartis, Pierre Fabre, Roche, Servier, Sirtex, and Taiho. JT reports consulting fees from Array Biopharma, AstraZeneca, Bayer, Boehringer Ingelheim, Cardiff Oncology, Chugai, Daiichi Sankyo, F Hoffman-La Roche, Genentech, HalioDx SAS, Hutchison MediPharma International, Ikena Oncology, Inspira, IQVIA, Lilly, Menarini, Merck Serono, Merus, MSD, Mirati, Neophore, Novartis, Ona Therapeutics, Orion Biotechnology, Peptomyc, Pfizer, Pierre Fabre, Samsung Bioepis, Sanofi, Scandion Oncology, Scorpion Therapeutics, Seattle Genetics, Servier, Sotio Biotech, Taiho, Tessa Therapeutics, TheraMyc, and Tolremo Therapeutics; honoraria from Imedex/HMP, Medscape Education, MJH Life Sciences, PeerView Institute for Medical Education, and Physicians Education Resource; and stock options in Oniria Therapeutics. KL, C-SS, and PB are employees of and own stock options in MSD. SYR reports research funding to their institution from Amgen, Astellas, Daiichi Sankyo, Eisai, Merck, Roche, Zymework, Indivumed, MSD, Ono, Bristol-Myers Squibb, and AstraZeneca; consultancy fees from Amgen, Astellas, Daiichi Sankyo, Eisai, LG Biochem, Indivumed, MSD, Ono, Bristol-Myers Squibb, and AstraZeneca; and fees for speakers bureaus from Lilly, Eisai, Daiichi Sankyo, MSD, Ono, and Bristol-Myers Squibb., (Copyright © 2023 Elsevier Ltd. All rights reserved.)