Covid-19 daily cases started rising again about 4 weeks ago signaling the beginning of a fourth wave in the USA. However, over the last week cases have resumed declining cutting short the fourth wave. Is this a real indication of light at the end of the tunnel or are we just seeing the headlight of the oncoming train?
42.2% of the USA has been vaccinated with at least one shot to date (green part of the graph below). This seems to be too low to meet the 70%+ threshold that is generally believed to be the vaccination level needed for herd immunity. However, we have always asserted that while rigorous herd immunity with vaccines is the most desirable goal, effective human immunity may be reached earlier and allow the USA to flatten the COVID-19 infection curve. Effective human immunity is a combination of natural immunity gained through infection by the SARS-COV-2 virus, vaccine immunity, and behavior immunity due to citizens rigorously practicing masking, social distancing, and isolation as necessary.
Natural immunity has been acquired by 32.8M Americans that have been confirmed as infected to date (red part of graph above). A much larger portion of Americans had been exposed to the virus but had asymptomatic or milder cases of COVID-19 who did not get tested because of lack of available and/or timely PCR tests (dark blue part of graph above). This could have been as large as 8X the confirmed population a year ago during the first wave when the USA was ill-prepared. Over time, this factor has declined with wider and more timely testing and we suspect it is just 2X confirmed cases now nationwide. Large studies have confirmed that natural immunity of 80% is likely for adults <65yrs old for at least 5 months. Several smaller studies have suggested even high levels of protection for longer periods of time. For modeling purposes, we assume that at least 60% protection is possible for 12 months. This model suggests that some 5M of the 90M Americans’ natural immunity is now too weak to protect them much. This group has also lost members because many of them have acquired vaccine immunity.
Behavior immunity helps to slow the spread of the disease (light blue part of graph above). Countries such as Taiwan and Vietnam have successfully used only behavior immunity to keep the SARS-CoV-2 at bay for well over a year. If everyone had masked, socially distanced, and followed all the other public health recommendations of the CDC, the USA would have reached effective human immunity sooner and avoided the fourth surge altogether. But too many states relaxed mitigation measures too soon after January. Fortunately, the pace of vaccination doubled from President Biden’s original goal of 100M shots in 100 days to 200M shots and compensated for this rapid relaxation. We believe that the USA may be reaching effective human immunity soon. Thousands of new cases will continue to be reported every day but the ability of the virus to spread exponentially will have been curtailed.
It may be too soon to celebrate because this effective human immunity is soft and require at least 60M Americans to continue to follow public health guidance rigorously. Over time, this soft human immunity will harden as vaccine immunity grows, even if at a slower pace than April due to vaccine hesitancy. New and more dangerous variants can still emerge and endanger this optimistic forecast, especially if international travel is relaxed too soon while the number of daily cases globally is still rising from surges in India, Brazil, and Turkey.