Extended Data Table 4.

Adverse events that occurred in 10% or more of participants in either group of the as-treated population

	Pembrolizumab group (N=217)		Placebo group (N=216)
Event — no.(%)	Any grade	Grade 3–5	Any grade	Grade 3–5
Diarrhoea	114(52.5)	16(7.4)	96 (44.4)	18 (8.3)
Nausea	106 (48.8)	10(4.6)	96 (44.4)	12 (5.6)
Anaemia	89 (41.0)	19(8.8)	95 (44.0)	20 (9.3)
Decreased appetite	67 (30.9)	5(2.3)	69 (31.9)	9 (4.2)
Vomiting	67 (30.9)	10(4.6)	59 (27.3)	4(1.9)
Platelet count decreased	53 (24.4)	17(7.8)	61 (28.2)	15(6.9)
Fatigue	51 (23.5)	9(4.1)	43(19.9)	6(2.8)
Neutrophil count decreased	51 (23.5)	16(7.4)	53 (24.5)	16(7.4)
Peripheral sensory neuropathy	50 (23.0)	6(2.8)	40(18.5)	3(1.4)
Aspartate aminotransferase increased	45 (20.7)	1 (0.5)	28 (13.0)	1 (0.5)
Weight decreased	41 (18.9)	2 (0.9)	37(17.1)	1 (0.5)
Palmar-plantar erythrodysaesthesia syndrome	40(18.4)	1 (0.5)	35(16.2)	2 (0.9)
Neutropenia	34(15.7)	11 (5.1)	33(15.3)	11 (5.1)
Constipation	33(15.2)	1 (0.5)	37(17.1)	0(0.0)
Neuropathy peripheral	33(15.2)	3(1.4)	35(16.2)	5(2.3)
Hypokalaemia	32 (14.7)	11 (5.1)	25(11.6)	14 (6.5)
Alanine aminotransferase increased	31 (14.3)	1 (0.5)	22 (10.2)	1 (0.5)
Infusion related reaction	31 (14.3)	4 (1.8)	20 (9.3)	2 (0.9)
Pyrexia	31 (14.3)	1 (0.5)	26(12.0)	0
Hypoalbuminemia	30(13.8)	0(0.0)	32 (14.8)	4(1.9)
White blood cell count decreased	27(12.4)	4 (1.8)	31 (14.4)	5(2.3)
Malaise	24 (11.1)	1 (0.5)	19(8.8)	2 (0.9)
Stomatitis	24 (11.1)	2 (0.9)	22 (10.2)	4(1.9)
Asthenia	22 (10.1)	5(2.3)	35(16.2)	8 (3.7)
Blood bilirubin increased	22 (10.1)	3(1.4)	12 (5.6)	1 (0.5)
Thrombocytopenia	20 (9.2)	7(3.2)	25(11.6)	2 (0.9)

The treatment regimen included trastuzumab and chemotherapy in both groups.