Analysis: Vaccines will reduce the risk of an MIQ breach but they won't make the border bulletproof, writes Marc Daalder
The positive coronavirus test result from an airplane worker who had been fully vaccinated has underscored the warning that vaccines are not a silver bullet against the pandemic, experts say.
While the Pfizer/BioNTech vaccine that New Zealand is using in its rollout is highly effective against the coronavirus, "there's no vaccine that is 100 percent effective," vaccinologist Helen Petousis-Harris told Newsroom.
"We should expect this, it's okay, don't freak out. We'll see some more of these," she said.
What is vaccine efficacy?
It all comes down to a single statistic: Vaccine efficacy.
(Actually, it's several statistics, but we'll come back to that later.)
You've probably heard that the Pfizer vaccine is 95 percent effective against Covid-19 - but what does that mean?
"The term efficacy comes from the clinical trials, in a clinical setting, where it's quite simply calculated as the number of persons who develop the condition over those who were vaccinated times those in the group that didn't receive the vaccine. It's a very simple measure, very straightforward, very clean," Petousis-Harris explained.
In other words, about 36,000 people in the Pfizer trial who hadn't previously had Covid-19 were given either two doses of placebo or two doses of vaccine. In the placebo group, 162 people - about one in every 113 - developed symptomatic Covid-19 at least one week after their second shot. In the vaccine group, that fell to just eight - one in every 2,500 people.
The chance of catching symptomatic Covid-19 in the control group was 0.88 percent while the chance in the vaccinated group was 0.04 percent - 95 percent lower.
So a 95 percent effective vaccine doesn't mean you'll have a 5 percent chance to catch Covid-19, it means that whenever you're in a situation where you normally would catch Covid-19, you're now 95 percent less likely to.
Other types of efficacy
The big Pfizer trials were mainly aimed at symptomatic disease, although they also found that just one person developed severe Covid-19 in the vaccine group, compared to nine in the control group. Further studies out of Israel and the United States have demonstrated that the vaccine is also effective against the virus in other ways.
"You see the words 95 percent and so forth bandied about ... and then there's the real world. And that's where you can see things behave a little differently because you've got all of these other potential things at play and you certainly haven't randomly selected your people getting the vaccine anymore," Petousis-Harris said.
"At the moment, those numbers are holding up in the real world, by and large, which is nice. There's been numbers bandied around, as well, about how effective the vaccine is at protecting against picking up the virus and carrying it for a brief time. In a situation like that, you might therefore assume you could get asymptomatic transmission. We've got some data on both of those now, on the protective effect on picking it up and becoming asymptomatic but positive and also against the transmission. That is actually up there in at least 90 percent."
A study of nearly 4,000 frontline healthcare workers in the United States before, during and after vaccination found the Pfizer and Moderna mRNA vaccines were 90 percent effective against Covid-19, regardless of symptom status. Israeli data with much larger sample sizes found the Pfizer vaccine was 94 percent effective against asymptomatic infections and at least 97 percent effective against symptomatic disease, severe disease, hospitalisation and death.
"That leaves some people who are actually going to be susceptible to these things. Almost nobody dies, almost nobody goes to hospital, but then you've got these other outcomes," Petousis-Harris said.
"We shouldn't be surprised - we are going to get cases in fully vaccinated people. You're just going to see the vast majority occurring in the unvaccinated, when they do occur. But I think we need to keep people aware that one in 10 or one in 20 people will still be not completely protected. We've got lots of virus out there and therefore, if you keep testing people all the time, there are going to be positives."
Covid and coronavirus
The airplane cleaner is the perfect example of this. In fact, technically speaking, he doesn't even have Covid-19, Petousis-Harris said.
"This guy as well, right, he's not sick. Covid-19 is the disease and the disease is where you're sick. SARS-CoV-2 is the virus that causes the disease. Being positive for the virus doesn't mean that you're sick," she said.
While Covid-19, coronavirus and SARS-CoV-2 have been used interchangeably in the media and by everyday people, the virus and the disease are in fact two different things. Covid is short for coronavirus disease - the disease caused by the SARS-CoV-2. Essentially, negative health outcomes from the coronavirus are the disease. If you're vaccinated and therefore don't experience negative health outcomes from the virus, then you won't be experiencing the disease.
That doesn't mean you can't pass it on, however. In fact, some experts worry that the vaccination of border workers could make border breaches more likely in some ways. If you catch the coronavirus and the vaccine prevents you from displaying symptoms, you may be more likely to escape detection.
Onward transmission
Researchers from Te Pūnaha Matatini (TPM) have modelled the impact of vaccinations on the border workforce. They say that a given infected person is now more likely to cause a significant community outbreak.
"Under a worst-case scenario of a vaccine that prevents symptoms but does not reduce transmission at all, vaccination of frontline workers could approximately double the risk of a large community outbreak," the study warned.
The problem is that we still don't know to what degree the vaccines might prevent an infected person from infecting others. Overall, it is likely to reduce the chance of onwards transmission slightly, if only because vaccinated people are unlikely to become symptomatic and the coronavirus spreads most commonly through droplets from symptomatic people.
The TPM researchers modelled the likelihood of onward spread from an infected border worker under a range of different scenarios. Without vaccines, there is a 6.8 percent chance that an infected worker spreads the virus to others. That more than doubles to 14.2 percent if workers are vaccinated with a shot that reduces symptoms but doesn't reduce the chance of transmission whatsoever. Even if the vaccine halves onwards transmission, undetected community spread from a given index case is still more likely (8.5 percent) than without vaccination.
The vaccination of border workers' families poses another issue, in that any potential outbreak could be much larger by the time it is detected. That's because the border worker is most likely to spread it to their family members, who would similarly be asymptomatic and then each spread it to yet more people.
While an outbreak is slightly more likely to be detected once the families are taken into account, it is also likely to grow larger if it escapes detection. For the aforementioned scenario where workers and their families are vaccinated with a shot that reduces symptoms but doesn't reduce onward transmission, the outbreak would involve 27 cases when detected instead of just 22.
Vaccines still reduce border risk
Of course, the chance of a given worker becoming infected in the first place plummets significantly with the vaccines. The study's lead author, University of Canterbury mathematics professor Michael Plank, offered very rough figures of the risk that a given border worker might become infected with the coronavirus. Based on eight MIQ workers testing positive in the past nine months, any individual MIQ worker has about a 0.24 percent chance of testing positive in a year.
From that "big simplifying assumption", Plank said, over the entire workforce, there's a 19 percent chance that an MIQ worker will become infected in any given week and a 59 percent chance in any given month.
With a 95 percent effective vaccine, those risks nearly vanish, down to about 1 percent a week or 4 percent a month.
"These are very simple calculations that are highly approximate and ignore lots of variables," Plank caveated, but they broadly show the impact of vaccines on the risk of future border breaches.
So vaccines reduce the risk of a border worker becoming infected, but could increase the risk of that infected worker spreading the virus to other people.
In order to further reduce that risk, the TPM researchers recommend increased testing of border workers, possibly through regular saliva testing. If workers are tested every four days instead of weekly, then the chance of undetected spread even among workers vaccinated with a shot that doesn't prevent onward spread falls from 14.3 percent to 5.3 percent.
"Our results should not be used to suggest that border workers should not be vaccinated as a priority. Protecting them from the health impacts of Covid-19 at the earliest opportunity is a national ethical obligation," the researchers wrote.
"However, care needs to be taken to ensure this group does not inadvertently become a silent source of transmission into a community that is largely unvaccinated at present."
