COVID-19 cases are falling. This could be the beginning of the end of the pandemic.

Robert M. Kaplan, Opinion contributor
·5 min read

In early January, as new COVID-19 cases were at an all time high, I published an op-ed predicting that new cases would soon decline and that the December-January surge was likely to be the last one. The column was not well received by colleagues whose models indicated cases would continue to rise throughout the winter. But, since mid-January, new cases have fallen precipitously. Most of the different explanations for the decline are probably incorrect. Here’s why:

Vaccines. Vaccines will be essential for ending the pandemic, but they don’t explain why cases are now quickly declining. Very few people had been inoculated when the decline started, and only more than 13% have received shots in the United States. Further, cases are dropping in countries that have initiated vaccination programs and those who have not started. Japan, where vaccinations only launched last week, had a 85% decline in new cases since Jan. 11. In Colombia, where vaccines are unavailable, new cases have fallen 78% since Jan. 20.

Weather, masks and herd immunity

Change in weather. Is the decline because spring is coming? Probably not. Cases began to fall in the midst of winter. As horrific storms battered most of the United States last week, it seems unlikely that people were outside enjoying the sunshine. If seasonal change is a cause, the pattern in the northern hemisphere should be opposite to that in the southern hemisphere. Yet Argentina, South Africa, Zambia, Zimbabwe, Australia and most other southern hemisphere countries are experiencing declines similar to countries in the northern hemisphere.

Masks. Most authorities agree that masks decrease the spread of virus. But do changes in mask wearing coincide with changes in infections? Stanford and YouGov have completed a series of surveys of representative samples of the U.S. population. In each, we asked respondents whether they "always" wore masks outside the home. Mask use increased from nearly 58% to 73.5% between May and August, but only increased from 73.5% to 74.5% between August and Christmas. Mask wearing was essentially unchanged between fall and the pandemic peak. The 1 point increase could not account for the precipitous rise and then fall in the number of cases.

Los Angeles health workers administer COVID-19 vaccinations on Feb. 24.
Los Angeles health workers administer COVID-19 vaccinations on Feb. 24.

Variants. While we celebrate the decline in cases, many fear that we will soon be buried in the avalanche of new cases caused by more dangerous variant strains of the virus. The highly contagious British variant is now doubling every 10 days and could become the dominant strain in the United States by the end of March. But the impact of variants has not followed predictions. Variants now account for a majority of cases in the United Kingdom and in South Africa. If these mutated versions can reinfect those previously afflicted or if they are evading vaccines, we would expect large increases in new cases in the United Kingdom and South Africa. But that is not happening. In both countries, the number of cases is rapidly declining.

Maybe the last siege: Brace for another onslaught of COVID-19 cases. The post-Christmas surge has begun.

Natural immunity. A final explanation is that we are running out of people who have not yet been infected. Once infected with SARS-CoV-2, natural immunity offers powerful protection. Although there are some cases of reinfection, they are extremely rare. Among 113 million confirmed COVID-19 cases worldwide, there are only 57 documented reinfections. The placebo group in the Johnson & Johnson vaccine study includes 2,030 people who had previously been infected. These individuals were better protected against clinical infection than those who got the vaccine.

Pandemic may be in last chapter

About two-thirds of the population needs to have neutralizing antibodies in order to reach herd immunity. Two-thirds of the 330 million people in the United States is about 220 million people. You can get antibodies by taking an effective vaccine. But, if you are one of the 29 million Americans who have had a documented case, you are probably immune. And those 29 million with confirmed cases are the tip of the iceberg.

The Centers for Disease Control and Prevention estimates that about eight people have been infected for each documented case. Other estimates show that the 8-1 ratio is too high and have suggested that it is really 5 to 1 or 6 to 1, while others say it could be as high as 10 to 1. Let’s use a conservative estimate of 6 to 1. If we multiply the 29 million known cases by six, it is possible that more than 170 million people are already immune. This could explain why early rates of infection in North Dakota, South Dakota, Nebraska and Iowa were followed by rapid declines in new cases and hospitalizations.

Predictable: The COVID vaccine system is unfair to those who need the shots most. We can fix this.

Why are we paying so little attention to natural immunity? Perhaps it is because natural herd immunity came to be identified with a political ideology that also advocated reckless behavior. There are very dangerous consequences to encouraging people to get infected as a pathway to immunity. But we should not neglect the possibility that herd immunity might already be taking hold.

To be clear, this is not the time to back away from vaccines, social distancing and mask wearing. Just as firefighters should not leave the forest while the fire still smolders, we need to double down on proven medical and public health tools until all embers are extinguished. But we should also recognize that the vaccines are arriving at a time when cases are rapidly declining. Medical historians offer several examples of vaccines and treatments that became available after a pandemic was on its way to resolution. When all is said and done, vaccines will deservedly be given credit. But it is not clear they should be given all of the credit.

Robert M. Kaplan is a faculty member at Stanford Medical School Clinical Excellence Research Center, a former associate director of the National Institutes of Health and a former chief science officer for the U.S. Agency for Health Care Research and Quality.

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This article originally appeared on USA TODAY: Herd immunity, vaccines, masks and distancing could end COVID pandemic