More than a quarter of the US population has now been fully vaccinated against Covid-19, amounting to some 75 million people who have received both jabs.
With the US, the UK and Israel vaccinating hundreds of millions of people, researchers are getting a better idea of the true real-world protection the vaccines provide.
Overnight the US government reported some vaccinated people, as expected, have become sick from the coronavirus with what's known as "breakthrough infections".
The country's Centers for Disease Control and Prevention (CDC) said about 5800 of the breakthrough infections had been confirmed among those who had been fully vaccinated. However the agency warned that reporting of such cases was uneven and incomplete.
Of the roughly 5800 post-vaccine cases, 396 – or 7 per cent – requiring hospitalisation and 74 people passed away. Meanwhile about 29 per cent of the breakthrough infections were asymptomatic.
As with the flu, people who get Covid-19 after being vaccinated are more likely to have a milder illness than unvaccinated people.
The CDC has set up a nationwide data base for reporting of breakthrough cases, but its initial data did not reveal any "unexpected patterns… in case demographics or vaccine characteristics", it said.
Early data a 'good scenario'
While there has been some confusion online about the risks posed by breakthrough infections with social media posts questioning the point of the jabs, epidemiologists says the finding is expected and largely good news.
Tara Smith, a professor of epidemiology at the Kent State University College of Public Health in Ohio, said it was in line with what drug companies would be hoping for.
"This is a really good scenario, even with almost 6000 breakthrough infections," she told NBC in response to the CDC data.
"Most of those have been mildly symptomatic or asymptomatic. That's exactly what we were hoping for."
Speaking to Yahoo News Australia, Flinders University vaccine expert Nikolai Petrovsky said "clearly no vaccine is 100 per cent effective".
He cautioned it was "early days" with the CDC data, but "this might just represent the five per cent difference between a 90 or 95 per cent protection".
"It doesn't show the vaccine is not working, that would be the wrong conclusion," Professor Petrovsky said.
Pfizer/BioNTech has said its vaccine has an efficacy rate of about 95 per cent while the AstraZeneca vaccine – which was originally due to make up a bulk of Australia's rollout – has a stated efficacy rate of about 15 to 20 per cent lower.
"We know when a trial generates a result like 95 per cent effective, that's not the real world," Prof Petrovsky said, noting clinical trials leave out certain types of people.
"We always know the real world effectiveness is always going to be lower... There will be people getting infected who have had the vaccine."
Are variants to blame for breakthrough infections?
Prof Francois Balloux, from the University College of London, tweeted breakthrough infections were not a major concern as long as they were not severe cases.
"Over the coming weeks and months, we can expect a large number of preprints and papers reporting 'breakthrough infections'," he tweeted earlier this week.
"A proportion of breakthrough infections are expected. Otherwise, vaccine trials would all have reported 100 per cent efficacy.
"Breakthrough infections are a trivial concern as long as they don't lead to serious disease in the infected, and onward transmission of the virus remains limited," Prof Balloux said.
Prof Petrovsky said scientists would be conducting much more research in the coming months in order to get a better understanding of Covid breakthrough infections and said virus mutations could be part of the picture.
"It may represent a start of a problem where the vaccine is not protecting against the variant viruses," he told Yahoo News Australia.
While scientists are investigating the role of variants as a factor in infections among the inoculated population, US experts say there is so far a lack of evidence that breakthrough cases recorded in the country are being driven by variants.
“Currently, there is no evidence that Covid-19 after vaccination is occurring because of changes in the virus,” CDC spokeswoman Kristen Nordlund said in a statement.
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