Covid-19 NZ: How common are Covid reinfections (and how bad are they?)

There have been more than 15,000 reports of Covid reinfections in New Zealand, raising questions about how robust our immunity is against the virus. American epidemiologist Katelyn Jetelina recently wrote this piece about what we know. It has been republished with permission.

reinfections and infections after vaccination are increasing. At the same time, misinformation about reinfection is at an all-time high. This leaves people with a lot of questions. Here’s (hopefully) some clarity and answers for you.

How common are SARS-CoV-2 reinfections?

Before Omicron, reinfections were rare. In the UK reinfections included about 1% of cases in April 2021. With the introduction of Omicron, reinfection rates increased rapidly to 11% of all infections. At the moment, reinfections make about 25-27% of the cases in the UK (Remember the UK follows antigen and PCR testing.)

Unfortunately, we don’t have a nationwide picture of reinfections in the US. Some local jurisdictions are closely monitoring the data. In New York, for example. 25% of cases this week were reinfections. The rate of first infection is 28 per 100,000 in New York compared to a reinfection rate of 5 per 100,000. Waves are still strongly driven by initial infections, but reinfections are increasing.

How do we get infected again?

Our immune system is complex and has multiple defenses, including antibodies, T cells and B cells† Reinfections can occur for several reasons:

The virus mutates to go around our first line of defense, the so-called neutralizing antibodies. Omicron continues to mutate to do this better and better. But more importantly, Omicron cannot escape completely. Here’s Why Immunity Still Can to prevent infection for some.
Antibodies decrease over time, so our first defense wall is getting shorter and shorter.
Some people do not get an immune response after a primary and usually mild infection.

So unfortunately, with more transmission, a rapidly evolving virus and a virus that has recently mutated to become less severe, we can to expect SARS-CoV-2 reinfections.

Who is most at risk of reinfection?

Thanks to the UK supervision data, we know that people are more likely to be re-infected if they:

are not vaccinated.
had a “milder” primary infection with a lower viral load.
reported no symptoms with their first infection.
being younger (although reinfections are) rising for all age groups in the UK).

Are reinfections common in other diseases?

Immunity from some viruses and vaccines, such as: the measles, can take decades. However, like SARS-CoV-2, other respiratory viruses mutate more quickly and the antibodies decline, so reinfections are more common. This happens with the flu RSV, and other coronaviruses. During a regular (US) flu season, for example, 1 in 5 people are re-infected. Although children and adults can be re-infected with RSV, reinfection is: associated with a milder illness.

Ella Bates-Hermans/Stuff

How common are reinfections?

How quickly can someone be reinfected?

An early 2021 study found that people have excellent protection against SARS-CoV-2 in the first 90 days after infection. In fact, the CDC still uses the 90-day window to exclude counting new positives as reinfections. However, reinfections are more likely to occur.

A study in Denmark found it that while pre-Omicron reinfections were rare (593 out of 4.4 million people), many occurred within 60 days of infection. Two additional studies (here† here) confirmed reinfections occurred as early as 20 days after infection during the first Omicron wave (re-infections remained rare; 1,739 out of 1.8 million people).

Reinfection can theoretically occur once the virus has cleared, but clearance varies from person to person. The virus can linger well beyond the contagious period. This means that you may test positive for a PCR test weeks after the initial infection, but this will be a reflection of the initial infection and not a new infection. However, testing positive for an antigen test a few weeks after the initial infection is a reflection of a new infection (assuming it is not Paxlovid rebound†

One could theoretically become reinfected week after week or month after month, but this would be: incredible rare because it presupposes no immune response after each subsequent infection.

In fact, I am not aware of a single case of rapid reinfection described. The cadence of reinfection in the future (every 6 months? every year?) is not clear, as we are at the mercy of time, viral evolution and booster rollout.

How serious are reinfections?

Even if our first layer of defense is down (causing reinfections), our secondary line of defense starts with preventing serious illness.

We continue to see this phenomenon at the population and individual level:

Population level: Population patterns show that serious diseases are becoming rarer with each successive wave. Illness is milder for Omicron than for Delta, but not enough to account for this pattern. Our immunity wall is building and causing welcome patterns such as a decline in deaths in South Africa.
Individual level: We see a lower severity of the disease in reinfections. Before Delta, a study in Qatar found that reinfections 90% lower risk of hospitalization or death than primary infections. Another study in the UK, reinfections were found to be associated with a 61% lower risk of death than primary infections. Those who had been vaccinated had a lower risk of serious reinfection compared to those who had not been vaccinated. In 2021, the UK found that the viral load considerable reduced after reinfections compared to primary infections, thus protecting against serious diseases.
More recently, a Danish preprint study evaluated reinfections after 1.8 million Delta and BA.1 infections. No reinfections resulted in hospitalization or death. Even more recently, a preprint from Qatar found that the effectiveness against severe reinfection remained incredibly high (97%) until June 5, 2022. (It is important to recognize that this study population was very young and very healthy.)

What about the Veteran’s Affairs (VA) reinfection study?

A few weeks ago, a now infamous VA preprint was released comparing the risk of poor outcomes (e.g. death, health problems) in people with reinfections with those with primary infections. The viral pre-print sent shockwaves through the mediabecause the study was widely misinterpreted to say that the health risks of reinfections are worse than the risks of primary infections. This is not what the study found.

The authors did not compare reinfections with the same person’s primary infection. Instead, they compared people with reinfections to a separate cohort of people with primary infections.

Because of this, we can only conclude that being re-infected is worse than not being re-infected, which is expected.

It’s also important to recognize that this sample was high-risk: the average person in the study was 62 years old, 25% smoked, 80-90% were unvaccinated, 30-40% had diabetes, and 19-26% had a heart condition. Similarly, 20% of those reinfected were hospitalized during the first infection. Of those who had three infections, 8.3% were immunocompromised (compared to 1.1% of the first infections).

It is necessary to assess the severity of reinfection among risk groups. And it’s clear that there are vulnerable groups of people who will get serious reinfections. But assuming that reinfections are more serious than primary infections for everyone is incorrect.

If I had a fever before the first infection (moderate illness), will my reaction be milder upon reinfection?

This is not very clear. a danish preprint evaluated BA.2 reinfections after BA.1 infections. The number of mild symptoms was higher, but the number of moderate symptoms (defined as flu-like symptoms) was lower. This was a very small sample size (33 subjects), so more evidence is needed.

What about long Covid after reinfection?

We urgently need more research on long Covid, and we need to sufficiently recognize it as an infection risk. I couldn’t find much about the risk of long-term Covid from reinfections. We can hypothesis lower risk given lower viral load, but this is an educated guess, and we don’t know how long covid will last or how to treat it.

What it comes down to:

There are numerous reasons why we should do our best to reduce SARS-CoV-2 transmission and prevent infection. Wearing masks, keeping up to date with vaccines, and improved ventilation will help. And ideally, no reinfections would occur. But we’re way past the point of zero Covidand reinfections can be expected, as with other respiratory viruses.

Vaccination and infection-induced immunity markedly reduces the severity of reinfection. Unfortunately, protection against serious reinfection is not guaranteed for high-risk groups.

Katelyn Jetelina is an American epidemiologist. This piece first appeared on her Share stack.