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When a new virus hits the scene, very few people (if any) are immune, which allows the virus to spread like wildfire. Once the majority of people have become immune to the virus, however, we tend to see an indirect protective effect to the population as a whole, this is referred to as “herd immunity.”
Herd immunity is beneficial in that it can slow the spread of a virus and offer some protection to those who aren’t yet immune. There are only two ways to achieve this herd effect – either a large portion of the population gets infected (and recovers!) or receives a vaccine. The number of individuals who develop immunity needed to attain social immunity can vary a bit, depending on how infectious a virus is. Researchers use the symbol “Ro” – which stands for “reproductive number” – when analyzing how contagious a disease is. For example, if, on average, people infected with a disease infect two other people, the Ro for that disease is 2. If those infected with a disease infect three other people, the Ro would be 3, and so on.
Ro estimates for SARS-CoV-2 are between 2-3. In comparison, the Ro for measles is a whopping 12-18, smallpox is 5-7, mumps is 4-7, and the flu is 1.4-4. For highly contagious diseases such as measles, 92-94% of the population needs to be immune (due to previous illness or vaccine) for herd immunity to kick in. For something like the flu, that number is more like 30-75%. With COVID-19, the current estimates to reach herd immunity range from 60-70% of the population. Based on this, without a vaccine, at least 197 million Americans need to become infected with, and recover from, COVID-19 in order to reach social immunity. With current positive US cases estimated to be just a bit over 1.5 million (as of 5/18/20), it would seem we still have quite a way to go.
Right now, the lack of testing and the idea that COVID-19 may have been circulating in the US for longer than we initially thought, make it extremely difficult to know if the current numbers of positive cases in the US are accurate. Adding to the complexity in attempting to estimate when we might hit the critical mass needed to achieve herd immunity is the fact that we don’t yet know if SARS-CoV-2 will present with different “strains” each year, like the flu does. There are already some indications that the virus is changing from the original strain detected at the end of last year. There could also currently be individuals in the population that may already have at least some partial immunity to SARS-CoV-2 due to previously becoming infected by other seasonal coronaviruses that have been infecting humans for decades.
If you’re expecting a vaccine to arrive quickly (ie, 12-18 months) to help speed up the herd immunity against SARV-CoV-2, you may need to be much more patient. Researchers have been looking at creating vaccines for other common human coronaviruses for the past 50 years, with no vaccines available yet. Vaccine development for more serious strains, with mortality rates considerably higher than the current SARV-CoV-2, such as Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV), have also so far proved unsuccessful over the past eight and eighteen years respectively. Unfortunately, at least at the moment, it seems highly unlikely we’ll see a safe, proven effective vaccine for SARV-CoV-2 in just a year.
All of these issues, combined with a lack of full understanding of the virus and measures such as social distancing, sheltering-in-place, and regularly wearing masks, present unique challenges for researchers to work through when trying to determine when, and if, we will be able to achieve herd immunity for SARS-CoV-2. As is true with just about any other virus we’ve encountered in our history, this new virus is likely to be with us for years, if not permanently.
In light of all this, it’s vital to ensure your immune system is as strong as it can be, by eating a healthy diet, exercising/moving regularly, getting enough sleep, and managing your stress. Whether it’s with a vaccine or not, herd immunity for SARS-CoV-2 will take time, so take steps now to maintain a healthy lifestyle and give your immune system the support it needs to be best prepared to do its job against COVID-19 – just as it does with any other virus we encounter during our lifetime.