Scientists are working extensively on developing models for the optimal rollout of a vaccine. Initially, the vaccine supply will be limited, hence, the right strategy on vaccine deployment would not only help end the Covid-19 pandemic quickly but also decrease the mortality rate.
The Global Polio Eradication Initiative (GPEI) was established based on the outcome of the smallpox eradication program. It was also instrumental in reducing the number of polio cases. Since the establishment of the GPEI, the number of polio cases has reduced by almost 99%. The case of Covid-19 mass vaccination is different and more complex. Howard Forman, Professor at Yale University, explained that the level of difficulty in making decisions, in this case, is high because such a large-scale vaccination has never taken place. Further, previous vaccines have either been tested for years or were introduced gradually which aided in making proper strategy.
Eva Lee, Director of the Center for Operations Research in Medicine and Health Care at the Georgia Institute of Technology, has developed vaccine deployment models for Zika, Ebola, and influenza, and is now working on Covid-19. She stated that unlike prior situations, developing a model for dispensing the Covid-19 vaccine is more complex. This is because SARS-CoV-2 affects older people or individuals with health problems (obesity, diabetes, or asthma, etc.) making them highly vulnerable. At the same time, the virus spreads rapidly through healthy young adults who are, of course, more likely to recover. Hence, while developing the optimal model for vaccine rollout, mathematicians are facing conflicting priorities. Should they prioritize preventing deaths or slowing down the rate of transmission?
Marc Lipsitch, a Harvard epidemiologist, believes that older people should be vaccinated first. A recent model based on the spread of Covid-19 in six countries—the US, Spain, India, Brazil, Zimbabwe, and Belgium, concluded that if the rate of mortality has to be reduced, adults over 60 years should be vaccinated first.
To build a formula for vaccine rollout, mathematicians have to consider several conditions – socioeconomic status, daily habits, age, virus reproductive rate of the virus, social distancing, etc. There are other challenges as well. Vaccines that require two shots, several weeks apart, involve follow-ups to ensure the timely administration of the second critical booster shot. Storage and transportation also add to the list of problems for vaccines that require extremely low temperatures.
Another critical factor for mathematicians is to understand how long the immunity lasts after infection. The family of coronaviruses (e.g., SARS-CoV-2 and common flu) has a relatively high mutation rate. This protects novel strains against the body’s antibodies. Such uncertainties make things more difficult for modelers. For the time being, researchers are assuming that persons who have been infected are immune to SARS-CoV-2.
Some other models have also been developed that are guided by concerns about equality, ethics, building trust, and the greater public good. Scientists believe that all of these models will help guide the deployment of vaccines in the uncertain early days and help in ending the pandemic.