In discussions about that ever-elusive herd immunity, the COVID-resistant population is divided between those who are vaccinated and those who've achieved a "natural immunity" by becoming infected with the virus.
Either way, the immunity stems from the human body creating antibodies against COVID. So the question naturally emerges: If someone has already become infected and then recovered from COVID, do they still need to receive a vaccine? After all, shouldn't their body already have those valuable antibodies?
As with many elements surrounding the pandemic, more research is required to reach a definitive answer. Pima County Health Director Theresa Cullen acknowledges it's a controversial topic, but recommends those who've already had COVID to still get vaccinated for the simple reason she says that the number of antibodies the body produces post-COVID can vary widely, and thus, the strength of a person's immunity can vary as well.
"Because COVID really hasn't been around that long, there's a lot of unknowns. If you look at what people like the World Health Organization and the Centers for Disease Control are saying, even though we believe 90% to 99% of people who get a symptomatic disease develop some kind of antibodies, the amount of antibodies is unpredictable," Cullen said. "It seems to be clear that people who were significantly ill develop a stronger antibody response than people who were either asymptomatic or very mildly ill. And that's an important factor, because the recommendation that you get immunized would depend on a high level of antibodies. The problem is, no one really knows what a high level is."
Because of the difficulty of tracing asymptomatic cases, estimates of their prevalence vary. Reports estimate that anywhere from 20% to 60% of COVID cases are asymptomatic, and state that the majority of COVID is spread is due to these asymptomatic cases. However, data does show that those asymptomatic people most likely did not develop a significant antibody response.
"Because of our inability to know on an individual basis what your response has been, we recommend immunization for people post-COVID," Cullen said. "It's a risk/benefit, and the risks with acquiring immunity from the vaccine are very minimal... I think the word I'd use is 'enough.' We know that you should get some natural immunity, but do you have enough natural immunity to prevent you from getting sick again? The point is to get enough immunity in your system that if you get exposed, your memory T cells are going to stop it."
Cullen herself contracted COVID in December, and still got vaccinated as well. Cullen also serves as a clinical associate professor in the University of Arizona's College of Medicine. She is a 25-year veteran of the US Public Health Service's Commissioned Corps, and previously volunteered to establish and operate a medical unit during the Ebola outbreak of 2014 and 2015 in Sierra Leone.
Cullen says you can speculate the immune reaction from COVID cases in the state by combining the number of people who've contracted COVID (nearly 1 million in Arizona) with the number of people who've been vaccinated (nearly 3 million). With these numbers combined, we approach the low end of population estimates for herd immunity, but we're not seeing that halt in cases, which may indicate naturalized immunity is not sufficient (as well as the fact vaccines are not perfect).
"For us from a public health perspective, it's a fine line. We want to encourage vaccination independent of previous illness or infection," Cullen said. "From a population perspective, we can't reassure individuals what their current antibody status is, even if they had an infection—and because of the variants."
Another question is the longevity of immunity. Again, research is still being conducted, but experts estimate vaccines can remain effective for up to eight months. And a study in The Lancet ("Incidence of SARS-CoV-2 infection according to baseline antibody status in staff and residents of 100 long-term care facilities") found that residents in long-term care facilities had antibodies "associated with substantially reduced risk of reinfection... for up to 10 months after primary infection."
There is discussion as to whether those who've had COVID need both vaccine shots, or just one, because they likely already have some immunity. Cullen points to her work fighting a hepatitis A epidemic in the 1990s. That hepatitis vaccine also required two shots. And while Cullen said they were able to almost entirely stop transmission of that epidemic after only one shot, health workers still gave two shots to ensure immunity.
"I don't want to encourage people to only get one shot, but I think the more people we can immunize with only one shot, the more likely we are to stop the transmission," Cullen said. "And there is no indication that the vaccine side effects are any different for people who've already had [COVID]. Now there are individual reports of people that have had COVID, and then get the vaccine and report that they feel like they have COVID again, usually for 12 to 24 hours. But the problem with that is there are people who get the shot who've never been known to have COVID and they report those same kinds of symptoms."
According to the Arizona Department of Health Services, daily COVID cases have been at a low plateau since early March averaging, about 700 new cases in the state per day. Compare this to December and January, which had more than 5,000 new cases per day in the state.
"It's really a scientific miracle in some ways, with what's happened with COVID, because of the rapidity with which we developed the vaccine, produced, released it and immunized people," Cullen said. "None of us have ever seen anything like his before, to go from scratch. But the key thing to halt the pandemic, we need to continue to get people immunized."