Coronavirus: Vaccine coverage of at least 50% needed
A vaccine against the coronavirus disease (Covid-19) needs to be administered to at least 50% of the population, even if it has 100% efficacy, to break the chain of infection and avert the peak of the pandemic, a new study has found.
The multi-author study, conducted by scientists in the US and published in the American Journal of Preventive Medicine, flags vaccine coverage and vaccine efficacy as the two key parameters to watch in setting a disease-prevention policy.
Vaccine overage refers to the percentage of the population administered the vaccine. Vaccine efficacy is the percentage reduction in disease incidence in a vaccinated group compared to an unvaccinated group.
Some are pushing for the introduction of a vaccine as quickly as possible so that life can return to normal. “However, we have to set appropriate expectations. Just because a vaccine comes out doesn’t mean you can go back to life as it was before the pandemic,” said lead investigator Bruce Y Lee at the City University of New York.
“It is important to remember that a vaccine is like many other products -- what matters is not just that a product is available, but also how effective it is.”
The study found the vaccine has to have an efficacy of at least 70% to prevent an epidemic and of at least 80% to largely extinguish an epidemic without any other measures.
Vaccine efficacy has to be at least 60% (when it prevents infection) to reduce the peak by >99% (i.e., number of new cases daily doesn’t exceed the initial number of cases on the day the vaccine is given) when the reproduction number is 2.5 and vaccination coverage is 100%.
This vaccine efficacy threshold rises to 70% when coverage drops to 75% and up to 80% when coverage drops to 60%. “When coverage drops to 50%, it is no longer possible to eliminate the peak, even when vaccine efficacy is 100%,” the study said.
As of August 25, there were 173 Covid vaccine candidates across the world in various stages of clinical trials, according to the World Health Organization’s draft landscape of Covid-19 candidate vaccines. The US, UK and the European Union block have made deals worth billions of dollars with companies such as AstraZeneca Plc, Moderna Ploc and Novovax Inc.
India has not pledged money to any vaccine candidate but has set up an expert panel to look into procurement. The country has two vaccine candidates undergoing trials while a third – the Oxford-AstraZeneca candidate – is set to be produced by the Serum Institute of India.
But questions of vaccine coverage and efficacy continue to dog experts and policymakers, especially in countries where poverty and lack of infrastructure make administering vaccine doses a challenge, or where active anti-vaccine groups spread misinformation.Certain vulnerable demographics – especially from marginalised communities – can be at a further disadvantage.
“To boost vaccination rates, both long-term and short-term interventions are needed,” said Brandon Yan at the University of California, San Francisco. “We need a concerted public health campaign that includes public health officials, health care providers and local communities, and reaches those groups most at risk for not getting vaccinated. And we need a proactive primary care outreach strategy to address patient concerns and provide information on how and where to get vaccinated.”