Vaccines that require two doses worry some experts
As a third company announced exciting results Monday for its COVID-19 vaccine, the public health world increasingly turned its focus to the daunting task of administering those shots to billions of humans.
Officials must manage immunization logistics on an unprecedented scale, decide who gets the shots first and overcome mounting antivaccination sentiment.
But one of the least-discussed hurdles lies in a more prosaic fact — that most of the front-running contenders require people to receive two separate doses each.
That's potentially a problem, given research has consistently found as many as 70 per cent of adults prescribed a multi-dose vaccine don't come back after the first injection.
If a similar pattern of noncompliance played out with COVID, it would be all the more difficult to reach the desired herd immunity, experts say.
“We typically haven't had mass vaccination campaigns for adults that required multiple doses,” said Dr. Kumanan Wilson, a University of Ottawa epidemiology professor and vaccine policy expert. “This is something that could be easily overlooked, but could be an Achilles heel in our program.”
Given the expected demand for the vaccines, the two-dose regimen is likely to be an issue for both the public and health-care providers, said Sherilyn Houle, a pharmacy professor at University of Waterloo who has studied the question.
Annual flu shots would seem to be the closest comparison, but involve only one injection.
“Additional staff may be required to accommodate the added workload to administer the vaccines, manage wait lists and appointments and monitor inventory,” Houle said. “I have heard some health professionals state that giving the COVID-19 vaccine is going to be like experiencing flu season two times over in terms of their anticipated workload.”
Having to administer twin doses will be “a challenge,” agreed Dr. Caroline Quach, a pediatric infectious disease specialist at the University of Montreal and member of the national advisory committee on immunization. But, she said, “it has to be done.”
Possible solutions are in the works, including a mobile app developed bywilson and colleagues that will send out text messages to remind people to get their second dose.
And one top expert said she actually does not anticipate a problem with Canadians getting their second dose, given that most recognize the seriousness of the pandemic.
A collaboration between Astrazeneca and Oxford University reported Monday that their vaccine was, depending on the doses administered, between 70 and 90 per cent effective at preventing COVID-19.
Their announcement followed on the heels of news from two rivals in the global coronavirus vaccine race — Pfizer andmoderna — that their products appeared to be 95 per cent effective in phasethree trials.
The promising results are better than expected. But all require two doses, with a gap of 21 days for Pfizer's product and 28 days for the other two.
In fact, of the seven vaccines in late-stage trials that the federal government has pre-ordered, only one, developed by Johnson and Johnson, needs just a single injection. It has yet to report any Phase 3 results.
Adherence to multi-dose vaccinations is relatively good for young children, who typically have regular medical appointments and are guided by their parents, notedwilson.
The research around adult vaccination is sparser but the findings have a consistent theme, the word “suboptimal” appearing in many of the papers.
A 2009 U.S. study, for instance, found that only 40-50 per cent of people completed two-dose hepatitis A and varicella (chicken pox) vaccinations, the numbers falling even lower for teenagers and young adults.
AU.K. study published last year found that a mere 11 per cent of adults got the required two doses of Hepatitis A vaccine within one year, the number rising to just 23 per cent by 36 months. A similar U.S. study in 2018 suggested only 32 per cent of adults had obtained their second Hepatitis A injection within 42 months.
People have told researchers they didn't know an additional shot was needed, couldn't fit it into their schedule or needed a reminder, said Houle, who helped conduct a similar study using Alberta data.
But Dr. Shelley Deeks of Public Health Ontario, another member of the national advisory council on immunization, said she's not overly worried, noting that Canadians tend to be more compliant with vaccines the more they fear the disease in question.
“I have heard some health professionals state that giving the COVID-19 vaccine is going to be like experiencing flu season two times over in terms of their anticipated workload.”
Sherilyn Houle Pharmacy professor, University of Waterloo