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(CNN)Results from trials involving three different coronavirus vaccines released Monday, all showing positive results, with evidence the vaccines can produce immune responses that would be expected to protect people against infection.
They all also appeared to be safe, although it will take studies with more people to show how safe they really are and whether they can prevent infection.
Early results of a closely watched Phase 1/2 trial published in The Lancet suggest a coronavirus vaccine developed by the University of Oxford and AstraZeneca is safe and induces an immune response. However, researchers stressed more study is needed to know whether the vaccine protects people against the virus.
Phase 2 results for one vaccine candidate made by Chinese company CanSino Biologics were also published in the medical journal The Lancet and early results from Phase 1/2 trials of the vaccine made by Pfizer and BioNTech were released in a pre-print paper that has not yet been peer-reviewed.
The Oxford vaccine prompted an antibody response within 28 days and a T-cell response within 14 days, according to the results published Monday. Neutralizing antibodies -- so-called because they can neutralize the virus -- were detected in most participants after one shot, and in all of them after two.
That's good news, researchers said. "The immune system has two ways of finding and attacking pathogens -- antibody and T cell responses. This vaccine is intended to induce both, so it can attack the virus when it's circulating in the body, as well as attacking infected cells," University of Oxford pediatrician Dr. Andrew Pollard, the study's lead author, said in a statement.
"We hope this means the immune system will remember the virus, so that our vaccine will protect people for an extended period. However, we need more research before we can confirm the vaccine effectively protects against SARS-CoV-2 infection, and for how long any protection lasts."
It's also not clear how well the vaccine would perform in older people who are more at risk of severe disease from Covid-19.
The vaccine trial included 1,077 people age 18 to 55 with no history of coronavirus infection and took place in five UK hospitals from late April to late May. Participants received the Covid-19 vaccine or a meningitis vaccine.
There were no serious adverse event related to the vaccine; fatigue and headache were the most commonly reported reactions. Other common side effects included pain at the injection site, muscle ache, malaise, chills, feeling feverish and high temperature.
'There's a long way to go'
These Phase 1/2 results were promising, researchers said, but large-scale trials are needed to determine whether the vaccine protects against the coronavirus.
"The key elements required to proceed to a Phase 3 trial are all there," Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, told the Science Media Center in the UK about the Oxford trial.
"The responses measured in the blood and the absence of serious harms indicate there is a possibility of an effective vaccine against Covid-19. It does not yet show that the disease is reduced or prevented, and this will not be easy to show until phase 3 trials have been completed in settings where the SARS Cov-2 virus is circulating at a high rate and people are getting clinical and severe disease."
A Phase 1 study typically studies a small number of people and focuses on whether a vaccine is safe and elicits an immune response. In Phase 2, the clinical study is expanded and the vaccine is given to people who have characteristics -- such as age and physical health -- similar to those for whom the new vaccine is intended, according to the US Centers for Disease Control and Prevention. In Phase 3, the vaccine is given to thousands of people and tested for efficacy and again for safety.
Phase 2/3 trials of the Oxford/AstraZeneca vaccine are currently underway in the UK, Brazil and South Africa, and later-phase trials are expected to start in the United States in August.
"This is a positive result, but again there's a long way to go. These are Phase 1 studies. We now need to move into larger scale real-world trials, but it is good to see more data and more products moving into this very important phase of vaccine discovery," Dr. Mike Ryan, executive director of the World Health Organization Health Emergencies Program, said during a briefing on Monday.
Oxford vaccine expert Adrian Hill said the team is looking at trying both one and two doses in Phase 3 participants.
AstraZeneca says it has secured capacity to produce 2 billion doses of the vaccine.
"If possible, there'll be a vaccine being used by the end of the year," Hill said.
There are 23 Covid-19 vaccines currently in clinical trials globally, according to the World Health Organization.
CanSino vaccine Phase 2 results published
More Phase 2 results published Monday suggested CanSino Biologics' vaccine was safe and created an immune response. The trial, conducted in Wuhan, China, in April, involved more than 500 people given high, low or placebo doses of the vaccine, the release said.
The trial found that 95% of the participants in the high dosage group and 91% in the low dosage group showed either T-cell or antibody immune responses 28 days after vaccination. In the high dosage group, neutralizing antibody responses were induced in 59% and binding antibody responses in 96%. Binding antibodies attach to a virus but don't stop it from infecting cells.
About half of people who got the low dose developed neutralizing antibodies.
Most adverse reactions were mild or moderate. Nine percent of the participants in the high-dose group had severe adverse reactions within 28 days, the most common of which was fever.
This vaccine uses a weakened human cold virus called an adenovirus to deliver genetic material mimicking coronavirus. So there is a danger that people who have been infected by that adenovirus in the past could have immunity that may "partially hamper the specific immune responses to vaccination." Compared to younger people in the study, older participants had a significantly lower immune response and tolerated the vaccine better.
"Since elderly individuals face a high risk of serious illness and even death associated with COVID-19 infection, they are an important target population for a COVID-19 vaccine," one of the study authors, Wei Chen, an expert on antibody engineering at the Beijing Institute of Biotechnology, said in a statement. "It is possible that an additional dose may be needed in order to induce a stronger immune response in the elderly population, but further research is underway to evaluate this."
In a commentary also published in The Lancet, Naor Bar-Zeev and Dr. William J. Moss of the International Vaccine Access Center and Johns Hopkins Bloomberg School of Public Health wrote that the trials were "broadly similar and promising," and the safety data "reassuring."
But questions remain about how long an immune response will last and how the vaccines will impact older populations, people with particular health conditions that put them at risk, and racial and ethnic groups more severely affected by Covid-19, they said.
"When things are urgent, we must proceed cautiously," they wrote. "The success of COVID-19 vaccines hinges on community trust in vaccine sciences, which requires comprehensive and transparent evaluation of risk and honest communication of potential harms."
More vaccine data from Pfizer, BioNTech
Also on Monday, US pharmaceutical company Pfizer and German biotechnology company BioNTech reported their Covid-19 vaccine candidate elicited a "robust" antibody and T-cell immune responses in an early Phase 1/2 study.
The data has not yet been published in a peer-reviewed medical journal, but was published in a pre-print paper to the online server medRxiv.org on Monday.
The findings mirror previous data among US participants in the study, which were announced earlier this month.
"It is encouraging that the data on BNT162b1 from the German study cohort are very much in line with what we have seen in the U.S. study cohort," Dr. Özlem Türeci, chief medical officer and co-founder of BioNTech, said in Monday's press release.
"The preliminary data indicate that our mRNA-based vaccine was able to stimulate antibody as well as T-cell responses at remarkably low dose levels," Türeci said. "We believe both may play an important role in achieving effective clearance of a pathogen such as SARS-CoV-2."
The German trial included 60 healthy adults ages 18 to 55 who were randomly assigned to receive varying doses of the vaccine.
The release also notes that there were some local reactions and mild to moderate adverse events, including some with flu-like symptoms and injection site reactions. All events resolved spontaneously and were managed with simple measures, according to the release. No serious adverse events were reported.
A Phase 3 vaccine trial that might involve up to 30,000 healthy participants is expected to begin in late July, if it receives regulatory approval, BioNTech and Pfizer said.
