How effective is the AstraZeneca COVID-19 vaccine? What you need to know

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Canada's National Advisory Committee on Immunization is giving the Oxford-AstraZeneca COVID-19 vaccine the green light to be used on adults who are 65 or older.

That decision comes two weeks after NACI said there wasn't enough evidence from clinical trials to show whether the AstraZeneca jab was as effective in older populations.

Health experts who disagreed with NACI's March 1 decision pointed to real-world data suggesting the shot was safe and effective among the elderly. Countries like France and Germany, who previously said they wouldn't offer AstraZeneca to those over 65, recently reversed their decisions.

Those countries, and several others around Europe, have since paused the use of AstraZeneca for another reason -- reports of blood clots among some recipients after receiving the jab. European regulators say there's no evidence the shot is to blame, however.

AstraZeneca's vaccine was the third to be approved by Health Canada, joining Pfizer-BioNTech and Moderna in late February. Canada has since authorized a fourth vaccine by Johnson & Johnson.

Here's what we know about the AstraZeneca vaccine:

HOW EFFECTIVE IS IT?

Data from clinical trials showed AstraZeneca was 62 per cent effective in preventing COVID infections, but it also prevented death and hospitalization in all participants who got the virus after receiving the vaccine.

Efficacy was a major talking point when AstraZeneca was first approved, with some comparing it to the 95 per cent efficacy shown in mRNA vaccine trials from Pfizer and Moderna. But experts have stressed that all the authorized vaccines offer excellent protection against severe disease.

Dr. Zain Chagla, an infectious disease expert with McMaster University, says when dosing regimens are factored in, AstraZeneca's efficacy rises to "70 to 80 per cent by the second dose, which is comparable to Pfizer, Moderna and Johnson & Johnson."

Real-world data is emerging that may also suggest the efficacy of AstraZeneca's vaccine increases over a longer time interval between the first and second shot. Clinical trials used a four-week span between doses but some countries have been delaying second doses by several weeks.

DOES IT WORK AGAINST THE NEW VARIANTS?

AstraZeneca's vaccine had some promising data last month suggesting it works against the variant first detected in the U.K. Findings based on swabs taken from around 500 volunteers in trials between October and January showed a 74.6 per cent efficacy rate against that variant.

A group of experts on immunization working with the World Health Organization recommended the use of AstraZeneca's vaccine in February, even in countries where variants have emerged as dominant.

That guidance came after a small study in South Africa suggested AstraZeneca's vaccine was only minimally effective against the variant first detected there, causing the country to halt use of the product.

WHAT ABOUT THE BLOOD CLOT ISSUE?

A number of AstraZeneca recipients in Europe have reported blood clots after getting the jab, casting doubt on its safety. But a European Union medicine regulator said Tuesday there was "no indication" that AstraZeneca's vaccines are causing the reported clots.

Dr. Lynora Saxinger, an infectious disease specialist with the University of Alberta, says a blood clot, or a pulmonary embolism, is "an incredibly common thing."

Saxinger said she would need to see more information about the blood clots that are being reported, such as how old the recipients are. But numbers of those reporting the clots should also be seen in the context of how many people get the vaccine and don't report that side effect, she added.

"We did not see any signal of (blood clots) in the trials of the vaccine, which had tens of thousands of people in them," Saxinger said.

Chagla says data suggests the number of those reporting blood clots is well within the normal range experts would expect to see in a given population, meaning they may have happened in those people whether they got the vaccine or not.

AstraZeneca said over the weekend that a review of 17 million patients who received the shot in Europe and the U.K. shows no elevated risk of blood clotting.

Prime Minister Justin Trudeau offered reassurances on the safety of the AstraZeneca vaccine on Monday, saying Health Canada regulators are constantly analyzing all the available information and have guaranteed that the inoculations approved in Canada are safe for use.

HOW DOES THE VACCINE WORK?

Unlike Pfizer-BioNTech and Moderna, which use messenger RNA (mRNA), the AstraZeneca vaccine is a non-replicating viral vector, using a weakened chimpanzee cold virus as a vessel.

Scientists stripped the genes from that virus, which isn't harmful to humans, and replaced them with the spike protein gene for SARS-CoV-2.

Once injected, the vaccine shows our bodies how to produce the immune response needed to ward off future infections from the COVID-19 virus.

Some may see outward signs of an immediate immune response to the vaccine -- the body's way of preparing for what it perceives as an attack by the virus. This can cause side effects usually seen with other vaccines, including pain at the injection site, redness, swelling and even fever, but experts say that means the vaccine is working.

WHAT ARE THE ADVANTAGES TO THIS VACCINE?

AstraZeneca can be shipped and stored at regular refrigerator temperature, unlike Pfizer-BioNTech and Moderna, which need colder storage temperatures.

From a global vaccination standpoint, the low cost of AstraZeneca's vaccine -- about US$4 per dose -- gives it another advantage. AstraZeneca, which says it aims to manufacture up to three billion doses in 2021, has pledged to make their product available at cost around the world until at least July.

-- With files from Canadian Press reporter Cassandra Szklarski

This report by The Canadian Press was first published March 16, 2021.