The outcome, though, will be at the lower end of its guidance of a decline in earnings of between 1-4%, said the UK pharma.
Achieving this target will also require a sustained recovery in adult immunisation rates, it added, and especially for shingles vaccine Shingrix.
Immunisation rates in the US had returned to prior-year levels in the last month of the quarter to end-September, GSK said though vaccines sales still fell 12%.
Revenues overall fell by 8% to £8.65bn, while operating profits dropped 13% to £1.87bn.
Adjusted earning per share fell 8% to 35.6p, which was well ahead of broker forecasts that beforehand had been for around 30p.
GlaxoSmithKline is developing a COVID-19 vaccine in partnership with French group Sanofi and earlier had reported that it expects first results in early December from a phase I/II study currently underway.
A phase III study should start before the end of 2020, it added.
Emma Walmsley, chief executive, said: “GSK has responded well to a challenging operating environment this year with disciplined cost control and strong commercial momentum in key growth products including Nucala, Trelegy, Benlysta, 2 drug-HIV regimens, Zejula, Shingrix and our priority Consumer Healthcare brands.
“This, combined with improving vaccination rates this quarter, means we are on track to deliver within our earnings guidance range for 2020.”
Walmsley added: “We are also urgently advancing possible COVID-19 Solutions with our partners, including clinical trials for antibody therapy VIR-7831 and three different adjuvanted vaccines. We expect to see data on all of these before the end of the year.”
Nine months revenue rose 2% to £25.4bn with adjusted operating profit flat at £7.1bn.
Shares eased slightly to 1,358p.