Classic “herd immunity” is achieved when enough members of a population get infected and acquire natural immunity so that too few uninfected members are left to spread the disease. The threshold percentage depends on how infectious the disease is and for COVID-19 it is not known but generally believed that 65%-75% of the population needs to develop immunity to stop this pandemic. For humans, there are two more methods to slow or stop a pandemic: behavior immunity and vaccine immunity. Governments can convince their citizens, or if necessary mandate, many public health policies that can mitigate the spread of disease. Keeping your distance, wearing a mask, selective isolation, and improving personal hygiene are things that cattle can’t do but humans can and should do. People in many countries such as Taiwan, New Zealand, Australia, and Vietnam have been able to control COVID-19 for over a year now without a vaccine or natural immunity. Even in countries with greater emphasis on personal freedom vs community health, enough segments of the population do follow public health policies and contribute to the overall “effective human immunity” of the population.
In the USA, behavior immunity has been unreliable but not totally useless. When people see confirmed cases in the USA reach a horrendous 250,000 per day shortly after New Year more take notice and reach for their masks. When they see 24M or 7.3% of their family and friends cumulatively get confirmed COVID-19 more may maintain social distance. When the new administration mandated mask usage at Federal facilities it improves behavior immunity. Near this level of infection, most people will have known at least someone within their immediate and one removed circle of family and friends that has had a confirmed case. It is hard to maintain COVID denial when among their 10 closest friends and family and their 10 closest friends (100 people) someone they trust gets COVID-19. Most people will modify their behavior – improving behavior immunity.
Moreover, the true level of community infection could be 2 (2.8 is our best guess for USA cumulative to date) to 8 times higher than that reported as many people get asymptomatic and mildly symptomatic COVID-19 that never got tested and confirmed. Everyone who gets COVID-19 develops certain amount of natural immunity depending on the severity of their case. Currently the small number of reinfection, ~50 worldwide out of 111M confirmed cases and certain specific studies suggest that natural immunity could be better than 95%. Adopting more optimal strategies of postponing or only giving one-shot vaccines to those who have had COVID-19 or postponing the second shot will allow us to reach effective human immunity sooner. Some have suggested that when the cumulative confirmed cases pass 11%, the true level of natural immunity in the community could surpass the 70% needed for COVID-19 herd immunity. This level was approached (9%) in North Dakota (ND) and South Dakota (SD) by Nov 15th and Nov 27th, respectively, and daily case counts peaked then. However, the fact that new cases continue to be reported every day in ND even as 13% of the population has now confirmed COVID suggests that the true level of total cases is not 6.5 times reported but much lower. Looking at highly infected communities such as Miami-Dade County in FL where 14.5% of the population has had confirmed COVID-19, antibody tests (a convenient but possibly incomplete measure of immunity) show only a 22% cumulative positivity rate, and cases have peaked but not stopped suggesting that the level of hidden cases is probably only 2 times confirmed.
Nevertheless, we may be closer to achieving effective human immunity than most are forecasting. In ND, a combination of vaccine (currently 15% at least 1 shot) and natural (currently confirmed 13% and estimated total 35%) immunity could soon end the pandemic even though only 15% of their population is estimated to adhere to CDC guidance for masks and social distancing. Similarly in Miami-Dade, effective human immunity is approaching 70% (n37%+v10%+b18%). For the USA as a whole, 8.6%-25% natural immunity currently plus 13% vaccine immunity and 25% behavior immunity may allow the country to achieve effective human immunity by the end of April (n29%+v26%+ b20% – see graph below).
This optimistic view may be undercut by communities rushing to reopen and removing mitigation measures before the contagion has fallen below critical levels. The other caveat is that if the more contagious variants take hold in the USA, another surge in cases may occur stretching out the time to effective human immunity. Even under these more pessimistic scenarios, the USA should achieve effective human immunity by mid-summer if vaccinations proceed at the current 25M people per month.