When Will Herd Immunity for Covid-19 be Achieved?

By Paul E. Lemanski, MD, MS, FACP

In the first five articles on Covid-19, I have stressed that the lifestyle approach of proven risk mitigation strategies – including masking, social distancing, handwashing and targeting quarantine – was essential to protecting us until natural infection (despite properly executed strategies) or preferably a vaccine had produced antibodies in the blood of a sufficient number of individuals to achieve herd immunity. To redefine that term: if you have no antibody protection from the virus, you need to be surrounded by individuals (the so called “herd”) who have antibodies to Covid-19 that protects the individual from exposure to virus. This is important to stop the transmission of the virus thru the population. 

Both the currently available Moderna vaccine and the Pfizer vaccine are effective and will protect 95% of the vaccinated population from developing symptomatic Covid-19. More importantly, they may protect and even higher percentage from severe disease or death. However, the exact percentage of individuals needed to achieve herd immunity with Covid-19 is not known; it is estimated to be in the range of 70-80%. The question remains: when will herd immunity be achieved?

The time to herd immunity is determined by the rate of vaccination and the rate of new infections of the virus by natural person-to-person transmission. The rate of vaccination, in turn, is determined by availability of vaccine and the number of individuals who actually receive it. As of Feb. 7, there has been widespread scarcity of vaccine with only 2.8% of Americans having received two doses of vaccine (the required number for 95% protection from symptomatic disease), and 9.5% having received at least one dose. The rate of spread of virus by natural infection is slowing a bit and to date there have been over 27 million cases in the US and 463,338 deaths. 

The rate of new infections is a function of how well people practice – despite increasing Covid fatigue and thus poorer compliance – the proven lifestyle risk mitigation strategies named above, and whether mutations in the virus (so called, new “variants”) are more easily transmitted. Assuming a best-case scenario of plentiful vaccine, a population willing to be vaccinated, and the logistics of getting folks in to receive vaccine optimized, I would, optimistically, guesstimate it will be end of summer to next winter before some “semblance of normality” returns. In the interval, the stricter the adherence to proven lifestyle mitigation strategies, the safer we all will be and the sooner “normality” resumes.

The “wild card,” of course, are the variants. First, the variants appear to be more easily transmitted, although – at least at this time – not more dangerous of causing more severe disease. Nevertheless, because they transmit more easily, more people will become infected and so proportionately there will be more hospitalizations and deaths before everyone can be vaccinated. While those surviving such a natural infection would have immunity, we do not know how long natural immunity may last. Preliminary information suggests at least three months. (It is recommended that all previously naturally infected individuals plan to receive a vaccination.) 

Second, the variants may partially escape neutralization by the vaccines currently available. Specifically-tailored booster shots – currently unavailable – may be needed to protect against such variants. As is apparent, there are a number of variables at play and the continued use of proven lifestyle risk mitigation strategies will remain the foundation in which vaccination will achieve the optimal population-based outcome.

If the Federal government increases the available vaccine stockpile and local governments remove artificial barriers to vaccine delivery, it may be possible to increase the vaccination rate and accelerate the dash to herd immunity. Moderna’s recent request to the FDA to increase the number of doses in each vial of vaccine from 10 to 15 is an elegant way to help. In my opinion, it does not help for health care providers to be threatened with legal action for vaccinating – by necessity – an occasional individual who falls outside of the currently approved for vaccination risk group. There have been reports of vaccine wasted because it was drawn up in a syringe and there was a no show with no readily appropriate individual to vaccinate. Under such circumstances, in my opinion, it is better to vaccinate a willing lower risk individual than waste vaccine.

Finally, the time to herd immunity will definitely be a function of the percentage of the population willing to be vaccinated. Some folks are suspicious of vaccines in general, some believe these particular vaccines have not been sufficiently tested, some want to wait and see if any unwanted side effects develop that were not reported in the original clinical trials. In a recent survey (and these rates seem to change daily), 51% of UN-vaccinated adults would get the vaccine, 26% would probably be vaccinated, 14% would probably not, and 10% would definitely not be vaccinated. If the percentage of folks needing to have antibodies in their blood to achieve herd immunity is 80%, we are getting close to possibly not achieving it.

Certainly, the decision to be vaccinated is a personal one. And there are always unknowns. But subjecting the Moderna and Pfizer vaccines to a risk/benefit analysis (the known risks and the known benefits), in my opinion, argues strongly for vaccination. I have been vaccinated. I encourage my family to be vaccinated. I encourage my patients to be vaccinated. The way I see it, this virus is so transmissible, that eventually everyone not vaccinated, will become naturally infected. Then it becomes a roll of the dice determining if you are an individual who will have severe disease or even death.

In summary, the time to herd immunity and the resumption of a normal daily life depends on many factors including availability of the vaccines, the logistics of vaccine delivery, the emergence of variants with resistance to vaccines, and the number of individuals willing to be vaccinated. Lifestyle based risk mitigation strategies remain the foundation for keeping all members of our population healthy pending the achievement of herd immunity.

Paul E. Lemanski, MD, MS, FACP is a board-certified internist practicing internal medicine and lifestyle medicine in Albany (centerforpreventivemedicine.com). Paul has a master’s degree in human nutrition, he’s an assistant clinical professor of medicine at Albany Medical College, and a fellow of the American College of Physicians.