Serology studies that look for evidence of past cases find that in most states, between 10 to 20 percent of the population had been infected by the end of 2020. But that estimate does not account for the worst of the winter wave, when a quarter-million cases were reported each day — which means the current figure could be significantly higher. Modeled estimates from data scientist Youyang Gu suggest that nationwide, about 28 percent of people may have been infected with the coronavirus.
And now vaccination is adding to those with immunity to the virus. Over 13 percent of people in the United States have received one dose of vaccine, and 6 percent are fully vaccinated. Those numbers will continue to grow rapidly: The number of people fully vaccinated is expected to double by early April, bringing coverage to 1 in 8 people. Some of those vaccinated will have also recovered from natural infection, so the total number of people immune will be somewhat less than the number of infections and vaccinations combined. But by early April, the number of people in the United States with immunity may approach 40 percent.
This level of population immunity slows down transmission and will buffer against a fourth wave of infections. The network of people vulnerable to infection now has many more dead ends than it did when the virus first emerged. New flare-ups will remain possible, but the balance is shifting in our favor. When combined with coronavirus control measures such as wearing masks and conducting activities outside, we can create the conditions for a more normal summer with camp, picnics and road trips.
Still, the pandemic isn’t over: Even a 40 percent level of immunity is far too low to allow us to drop all precautions. Estimates of the population immunity required for transmission to splutter to low levels are around 60 to 75 percent, far from current coverage. Even at 75 percent immunity, the virus would not stop spreading: It would just slow down to sporadic outbreaks, which could be managed through more targeted control measures, rather than sustained, widespread transmission. We aren’t there yet — and we may never get there. More vaccinations will help inch us closer, but the truth is that the long-term future of our dance with the virus is uncertain.
Even when there is enough vaccine supply for everyone who wants a shot to get one, the number of people who want one may remain too low to help extinguish the virus. Each year, only about 45 percent of adults get the flu vaccine, which would be far from enough to keep the virus suppressed. Although polls show that willingness to get the coronavirus vaccine is higher than that, it still may not reach 75 percent among adults. And the vaccines are not authorized for use in children under age 16 yet, leaving a large proportion of the population ineligible, and at risk, for at least the coming months.
The duration of immunity is not well understood, either. It may be that immunity grows weaker over time, which would put high levels of population immunity further out of reach. The Centers for Disease Control and Prevention recommends that people who have been vaccinated be exempt from quarantine, but only in the three months after the second dose. That window may be extended as more data becomes available, and studies are underway to learn more about how long immunity lasts. For now, scientists remain cautious about whether one vaccine course will provide long-term protection. If regular boosters are needed, it will be challenging to ensure everyone remains protected.
In the end, it may be variants rather than waning immunity that keeps us from getting to herd immunity for real. Two variants of concern pose a problem right now. B. 1.351, first identified in South Africa, and P. 1, first identified in Brazil, may be different enough from the current strains that the vaccines are no longer a great match. (The two mRNA vaccines approved by the Food and Drug Administration are thought to provide some protection.) Disease surveillance data suggests these two variants are circulating at very low levels, with around 50 cases reported combined across the United States. If these variants become more common over time, they could sidestep existing population immunity by causing reinfections or cases in people who were vaccinated. Other variants, like those recognized in New York and California, are of unknown significance but are starting to gain attention.
Fortunately, scientists have already begun work to update the vaccines to match these variants of concern so they can be put to use if needed. And the FDA has said drug manufacturers won’t need to go through time-consuming clinical trials for tweaks to existing vaccines. But over the long-term, variants will continue to be a problem. We will need robust scientific infrastructure to find and track new mutations of the virus and update the vaccines to match. An additional $1.75 billion for genomic surveillance in the supplemental funding package being considered by Congress will bring the resources needed to make this possible, which is important to get us through this pandemic and help to guard against future threats.
We may not be able to end the pandemic through population immunity alone. But if we continue to wear masks, social distance, avoid risky indoor gatherings, and get vaccinated when eligible, we can get to a new normal. And we’re on the right track. New variants may complicate things, but we can put the tools in place to stay ahead of the virus.