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As the rest of the world has hunkered down to weather the COVID-19 pandemic, Sweden has remained remarkably open. The government has taken steps like issuing guidelines for Swedes to stay home, banning gatherings of more than 50 people and closing museums. Yet restaurants, schools and parks remain open. Consequently, Sweden’s ambassador to the U.S., Karin Ulrika Olofsdotter, said the country’s capital, Stockholm, could reach herd immunity sometime this month.
Herd immunity is how a society collectively fights off infections to keep the transmission of disease low. The Swedish approach may sound appealing in countries like the U.S., where lockdown measures have crippled the economy. But remaining open has had costs, with elderly people making up 86 percent of Sweden’s more than 2,000 deaths so far. And scientists are still unsure whether individuals who have recovered from the novel coronavirus are immune to further infection — much less whether entire populations can keep the disease from reaching their most vulnerable members.
Experts agree that a COVID-19 vaccine, while at least several months away, is our best chance to safely achieve herd immunity and minimize infections. Without a vaccine — and proper social distancing measures to break the chains of transmission in the meantime — experts fear hospital systems could be overwhelmed and many more people could die.
How Does Herd Immunity Work?
On an individual level, our bodies build up immunity by producing antibodies that recognize and fight off an infection from invading pathogens such as SARS-CoV-2, which causes COVID-19. Your body can build up these antibodies naturally after you are exposed to and sickened by a virus, or you can receive a vaccine that elicits the same antibody response without the infection.
Immunity often means you’re protected from a repeat infection; plus, you won’t pass the disease to others. Someone who is immune to measles, for example, won’t unknowingly spread the disease. Instead, most viruses spread when an infected and contagious person comes in contact with another person who is not immune to that particular virus.
As the name suggests, herd immunity looks at protection from a particular disease at the population level. The more people who are immune, the fewer people a virus can jump to, and the fewer it can infect.
“Herd immunity is the status of all the people living in a local area and their ability to fight off a given infection,” says Jaquelin Dudley, associate director of the John Ring LaMontagne Center for Infectious Disease and professor at the University of Texas at Austin. “It has to do with the transmission ability of a given virus.” If enough people in a given population are immune to the virus, it won’t be able to spread those who are still vulnerable.
For example, if 90 out of 100 people in a room are immune to measles, the other 10 people are not likely to get the disease even if they aren’t immune, Dudley explains. The antibodies of the majority of individuals, built up either via exposure or vaccination, protects the remaining vulnerable people. But if the number of immune people drops to 80, the remaining 20 will have a higher chance of contracting measles. With the novel coronavirus, experts predict at least 70 percent of the population will need to be immune to the virus in order to achieve herd immunity.
We don’t yet fully understand how SARS-CoV-2 spreads from person to person. But very few viruses have human “carriers,” who transmit the virus for months or years with or without symptoms, Dudley says. More often, an infected person has a short window where they could pass the disease to another individual, ranging from a few days to a few weeks. With SARS-CoV-2, an unknown number of cases is spreading via infected individuals who show little to no symptoms of the virus — just another reason to practice social distancing while waiting for a vaccine to be available.
“Viruses are going to spread, that’s what they do, and so what you want to do is bring that number down” through either immunity and vaccination or social distancing, Dudley says.
One of the big open questions right now is whether recovered coronavirus patients are immune from contracting the disease a second time. “We don’t know yet if having the virus protects you from getting it again,” says Jared Baeten, a professor of medicine and global health at the University of Washington. The World Health Organization has emphasized that we do not know if people who recover from COVID-19 are capable of getting sick again with the virus. “Individual immunity is not yet proven, much less herd immunity,” Baeten says.
Think of common respiratory viruses that cause the flu or the common cold — you may receive vaccinations every year and still come down with one of these viruses in the winter, says Baeten. You aren’t perfectly protected against them, as there are literally hundreds of viruses that cause colds, but having been exposed to at least one strain may make subsequent infections less serious. Immunity can also wear off over time, which is why booster shots are common to prevent infections such as tetanus. Other shots generally last for life, like the MMR vaccine for mumps, measles and rubella.
If a COVID-19 infection offers partial protection and we are able to develop a vaccine that is also partly protective — as is the case with the flu — we could still achieve a form of herd immunity, Baeten says. But despite the lingering uncertainties, both Baeten and Dudley emphasize that a vaccine will be crucial to achieving widespread protection from SARS-CoV-2 in the future.
In lieu of widespread natural immunity or a vaccine right now, social distancing has performed the same function, breaking those chains of transmission in order to prevent hospitals from becoming overwhelmed, says Dudley. Thirty-one states are set to relax social distancing orders this week, and a vaccine remains at least months away.
When a vaccine does become available, we will only be able to achieve herd immunity if the vast majority of people get vaccinated, Baeten says. Dudley agrees: “The vaccine is our best hope to improve herd immunity [and boost] the number of people that can resist the infection when they’re exposed,” she says.