By John D. Loike, Ph.D.
Until the Delta variant of COVID-19 appeared, scientists assumed that herd or community immunity could be achieved by simply vaccinating the majority of the population. However, the fact that the Delta virus can replicate in both vaccinated and unvaccinated individuals challenges this assumption. At the very least, we may have to mandate wearing masks as an additional safeguard to terminate this pandemic.
Herd immunity is achieved when a population can no longer spread an infectious disease because the individuals within this population are protected against an infection. Traditionally, vaccinations alone have been sufficient to achieve herd immunity. Immune protection can occur by either surviving natural infections or by receiving vaccinations. With many viral infections, such as polio and smallpox, we have achieved herd immunity via vaccinations. However, the data regarding the Delta variant, suggests that vaccinations may not be sufficient to achieve herd immunity because this variant can replicate and spread in vaccinated people.
In a study posted online in July of 2021, researchers in China reported that viral loads in individuals infected with the Delta variant were around 1,000 times higher in their respiratory tracts than in those infected with other variants. Thus, individuals infected with the Delta variant can expel more virus particles into the air and promote viral spreading more easily. As long as the virus can replicate in a human host it can mutate and stop our efforts to achieve herd immunity. Professor Sir Andrew Pollard, who helped develop the Oxford University/AstraZeneca vaccine said that reaching herd immunity is ”not a possibility with the current Delta variant.” Thus, scientists must assess whether we need to take additional actions to stop or severely limit the accessibility and replication of the virus in an in-vivo environment (i.e., human beings).
Currently, there are three scientific considerations that may improve our chances to achieve herd immunity by reducing viral infections. First, will we need to achieve a higher percentage of people vaccinated? Second, will booster shots from another vaccine type reduce viral infectivity and third, will we need to change the nature (i.e., protein target) of the vaccine to be more protective against the Delta variant. Until those assessments are completed, wearing high-quality masks might be advisable and a necessary response to combating the Delta variant. Michael Osterholm, director for the Center for Infectious Disease Research and Policy at the University of Minnesota, told PBS this week that people should not only wear masks but even upgrade from their cloth masks and bandanas to more heavy-duty masks — like N95s and K-N95s to end this pandemic.
There is evidence that masking can help achieve herd immunity. California’s better-than-average vaccination rates and newly implemented mandatory mask policies in parts of the state are thought to have reduced the spread of the Delta variant. California is reporting 141.1 new coronavirus cases for every 100,000 residents over the last seven days. In contrast, Texas and Florida reported 297.8 and 653.8 new cases for every 100,000 residents, respectively. Highly recommending or mandating masks in indoor locations can only be an added value to achieve herd immunity for COVID-19. Many localities, like Philadelphia and Oregon, are mandating masks for vaccinated and unvaccinated people for all indoor businesses. Whereas Texas is banning mandated masking as the state is surging with new COVID-19 infections.
Because polio is less contagious than COVID, herd immunity was achieved when 80-85 percent of a population was vaccinated. Measles is more infectious than polio and requires about 95 percent of the population to be vaccinated to achieve herd Immunity. In both cases, vaccinations alone appear to be sufficient to achieve herd immunity.
A critical question is why can we achieve herd immunity with measles and not with COVID-19. Both viruses are essentially transmitted via aerosols and air droplets. Both have high rates of transmissibility with high R0, or “R naught values [a mathematical term that indicates how contagious an infectious disease is]. One reason may be that for many viruses infectivity is usually associated with clinical symptoms, whereas COVID-infected people can transmit the virus to others even without knowing that they have COVID-19 and without exhibiting any clinical symptoms. In addition, breakthrough infections for measles and polio are much less frequent than for COVID-19 because those who are vaccinated rarely interact with infected people.
Science has made rapid achievements in the development of COVID-19 vaccines. The two-dose vaccines are highly protective in preventing people from contracting serious COVID disease, as reported in a July 2021 study published in the New England Journal of Medicine. Moreover, a recent UK vaccine trial studied the use of different combinations of approved COVID-19 vaccines for first and second immunization doses and found that mixing vaccine types provide better protection against the Delta variant. As more communities and countries are vaccinating against COVID-19 and monitoring the spread of this virus and its variants, scientists will gain a better understanding of whether giving COVID-19 booster shots or giving one vaccine type as the first dose and a second vaccine type as the second dose will lead to herd immunity. We may also need to genetically modify the COVID-19 vaccine to target the new variant. Until we learn more about the effectiveness of mass vaccinations, booster shots or modifying our current vaccines protocols, it is prudent that we institute wearing masks indoors.
John D. Loike is biology and bioethics professor at Touro College and the Interim Director of Medical Ethics Program at New York Medical College, a member of the Touro College & University System.