4 coronavirus reinfection cases have been reported in the Netherlands, bringing the total to 6 worldwide

• Several cases of coronavirus reinfection have been documented this week. The first showed up in Hong Kong, then another was documented in Brussels, and now a total of four more reinfection cases have been confirmed by health authorities in The Netherlands.
• These reinfection cases demonstrate how immunity to the novel coronavirus is somewhat transient, especially with mild infections.
• Even though coronavirus immunity isn't perfect, there's evidence here that even a mild initial infection can help our bodies combat the next one.
• In the same way, even if COVID-19 vaccines aren't 100% effective, they could still help stamp out the pandemic by providing enough people with enough immunity at the same time.

After scientists in Hong Kong reported the world's first confirmed coronavirus reinfection case on Monday in a 33-year-old man, a handful of similar data points began to trickle in, all pointing towards a trend: coronavirus infections don't provide everyone with perfect immunity against future illnesses.

In the Netherlands, Harald Wychgel, a spokesperson for the National Institute for Public Health and the Environment, told Insider that at least four cases of coronavirus reinfections have been confirmed with genetic sequencing in that country alone.

Another confirmed case was announced in Belgium, late Monday, shortly after the Hong Kong case was first made public.

Many of these reinfection cases share common traits: they have been mild, or even asymptomatic, suggesting that reinfections are unlikely to be severe, and that a person's initial illness, even if it doesn't protect them completely from a re-infection, can help shelter them from some of the virus's harshest effects, if they get sick a second time.

Four Dutch reinfection cases were reported, all in older adults

All four of the Dutch reinfections appear to have been mild cases, even though they've each been documented in patients over the age of 60, who tend to have some of the harshest coronavirus illnesses. The lapses in time between the first and second infections in patients in the Netherlands range from weeks to months apart.

Dutch Virologist Marion Koopmans announced Monday on public broadcaster NOS that her labs had confirmed one initial coronavirus reinfection case in the Netherlands.

Erasmus MC, where she works, told Insider that the patient in question was elderly and had pre-existing conditions.

Wychgel said another Dutch reinfection case has been confirmed in a nursing home patient. The person developed a second coronavirus infection roughly two months after their first, and the patient is still alive today, Wychgel said.

'The whole Hong Kong case put everything in a rush'

Dr. Jean Luc Murk, a clinical microbiologist in the southern Dutch city of Tilburg, saw the remaining two of the country's four confirmed cases of reinfection at his hospital. Both were in men over 60, and both reinfections were mild. The men are both still alive. He's hoping to submit papers about their cases to scientific journals at some point next week, but the writing and analysis are not quite ready yet.

"The whole Hong Kong case put everything in a rush," he told Insider on Wednesday. "We only began recently with the sequencing."

Because of Dutch privacy laws, Murk doesn't want to reveal too many details of the two cases he's researching publicly, but here's a glimpse at what the reinfections he saw were like:

The first was an elderly patient who "needed a little bit of oxygen," Murk said, during his first course of infection in the spring, but he recovered well, and was able to be discharged quickly. Just weeks later, the man returned to the hospital. He was having some difficulty breathing, and "it appeared he had developed profuse diarrhea," Murk said.

The patient tested positive again for COVID-19, the illness caused by the coronavirus, roughly three weeks after his initial diagnosis.

"He had tested negative in between, but still, that is no reason to think of this as a second infection," Murk said.

The doctor knew that people often test positive for COVID-19 for many weeks, months even, after an initial infection. It doesn't necessarily mean they have a second case, but rather that there might be some lingering dead virus in their body, or that their first infection is still waning.

But in this case, national labs conducted genetic sequencing of both the man's first and second viral infections, and found "at least six nucleotides that were different between the two episodes."

That's how Murk knew this was a bona fide second infection.

"Apparently, within this period of three or four weeks, this patient had contracted a second variant of the coronavirus," he said.

Murk has a couple different hypotheses for why this might be the case.

First, both the man's initial and second infections were relatively mild. It could be that the patient's body didn't produce a strong enough viral response to his illness the first time around to protect him when presented with another variant of the same illness weeks later.

Secondly, because both illnesses elapsed within a span of just a few weeks, it could be that the patient's body simply didn't have enough time to mount its proper viral defense by summoning T cells and antibodies to fight off another course of disease.

"The whole story could change if we find that he did have neutralizing antibodies in the period in between," Murk said. "We still do not know."

Another case could be considered a 'double infection' or rapid reinfection

The second case the doctor saw was a slightly younger man, in his 60s, who came to the hospital in the spring with a "typical" COVID-19 pneumonia, Murk said. He was quickly discharged.

But several days later, the patient returned to the hospital, this time with respiratory failure.

"The first thought was 'Ok, this is still the same virus,'" Murk said.

When the patient's viral illnesses were sequenced in the lab, the phenomenon was almost identical to the first re-infection case Murk saw: several different nucleotides appeared in the genetic code of the second illness, which let the doctor know this was another reinfection.

"This could perhaps be considered as a double infection, or a very rapid reinfection," he said.

Reinfections are no reason for panic

Just because a few COVID-19 reinfections have started to crop up among more than 23.9 million documented cases of the coronavirus worldwide doesn't mean that an initial infection does nothing to protect people from future illnesses, or that a vaccine won't help stamp out the pandemic.

The same phenomenon seen in these reinfections could hold true with coronavirus vaccines once they're developed. Even if vaccines don't fully protect people from infection, they could help our immune systems battle the illness better if confronted with it a second time.

Aylin Woodward contributed reporting to this story.