The trial in the USA remains on hold as United States regulators probe serious UK side effect.
Pfizer and BioNTech's vaccine is one of 35 potential inoculations now in some phase of clinical testing, according to the WHO.
The university said in large trials such as this "it is expected that some participants will become unwell and every case must be carefully evaluated to ensure careful assessment of safety". Being alert to potential adverse reactions with any new vaccine is part of the process, he explained.
"It's really one of the safety valves that you have on clinical trials such as this", he told "CBS This Morning".
He added that the spread of the pandemic around the world has caused great confusion and put health authorities in a hard and testing challenge, as the greatest difficulty came from making crucial decisions that enable the health care system to deal with the new reality, enhance its ability to control and contain the virus and reduce deaths.
The Spanish study is part of mid-stage, or Phase II, trials of the vaccine that are being carried out in three countries, also including the Netherlands and Germany.
Oxford said that globally some 18,000 people have received its vaccine so far.
The new trials of the vaccines will see volunteers receive aerosolised vaccines through a nebulizer, delivering the vaccine as airborne droplets through a mouthpiece.
Previous studies suggest that lower doses might be required than in intramuscular injections to give protection, the Imperial researchers said. The trials also assess effectiveness by tracking who gets sick and who doesn't between patients getting the vaccine and those receiving a dummy shot.
Britain has secured access to up to 190 million doses of a potential coronavirus vaccine from Valneva in deal worth as much as 1.37 billion euros, the French-Austrian vaccine firm said Monday.
"What we're going to be doing is targeting the vaccine to select groups of people who are at very high risk of a bad outcome from COVID to try to reduce their risk", he said.
The World Health Organization has cited AZD1222, replication-deficient chimpanzee viral vector based on a weakened version of a common cold virus which contains the genetic material of the SARS-CoV-2 virus spike protein, as a leading contender in the race for a vaccine. "In the meantime, the key continues to be our behavior".