As New Zealand navigates a path forward into a more open-border Covid world, antibody testing could play a useful role in staying on top of the virus – especially with vaccination and infection monitoring, writes Chris Pemberton
New Zealand has been fortunate to avoid large-scale, unmanageable outbreaks of Covid-19 that have occurred overseas. However, it has become clear that current strategies will need to be refined or revised to cope with the changing landscape in an era of mass Covid vaccination.
Prevalence of Covid in the NZ population
At the present time there are some 2900 cases of clinically confirmed Covid in New Zealand. The vast majority of these diagnoses have been supported by a lab result known as the nasal swab PCR test. In some cases, a separate blood test known as a serology/antibody test has been used to determine if there is a historical component to the case.
There is a major difference in the applicability of PCR versus antibody tests. PCR tests are used to confirm an active infection where someone may have symptoms consistent with Covid; in contrast, antibody tests can only be used several weeks after symptoms have resolved as it takes time for antibodies to the virus to mature and they cannot be used to diagnose an active case.
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However, we know from overseas studies that PCR tests do not provide an accurate picture of the total population infection rate. One way to gauge a better picture of just how many people have been infected with Covid is to do a population antibody survey.
Like all testing, antibody surveys and the statistics that come with them have their imperfections, but they are still a useful tool that can assist public health strategies and planning, especially around estimation of risk to patients, health infrastructure and health providers.
What we know from antibody studies in NZ and overseas
Antibody tests are not routinely available in New Zealand (they are only used in selected cases as described earlier), but two focused research studies (one in Southland and one in New Zealand blood bank donors) have suggested an infection to known (PCR-proven) case ratio of up to 2.3, suggesting that – as of April 2021 – there could be up to double (ie. ~6000) the known historical and active Covid cases.
However, the authors of both studies wisely urge caution in the interpretation of these results given the highly selected nature of the populations sampled. Overseas antibody studies have suggested infection-to-case ratios that range anywhere from two to 20, depending on the type of study and population sampled.
India recently completed their fourth population sero-survey which suggested that two-thirds of the population aged over six have antibodies against Covid, which is remarkable given 62 percent of participants reported not having been vaccinated.
New Zealand could likely benefit from similar antibody surveys to inform Covid management policy and practice, especially as we roll out our vaccination strategy.
The CANTATA Study
At the Christchurch Heart Institute, University of Otago, Christchurch we are embarking on a MBIE-funded study – known as CANTATA – which aims to answer three questions:
1) What is the proportion of people in the general population who may have antibodies specific to the Covid virus?
2) How do people’s antibody levels track during the course of vaccination?
3) How long do measurable antibodies last in the blood of those who have had Covid?
In order to carry this out, we have developed our own specific tests for the different Covid antibody types (we can distinguish Covid infection versus vaccination only antibodies) and we will be comparing them with proven commercially available tests that have received regulatory approvals overseas.
Further to this, we will follow the heart health outcomes of those who have had Covid and those who have been vaccinated compared to those in whom neither has occurred, in order to gain some understanding of the medium to longer-term consequences of this infection or vaccination upon the heart.
Information gained from the study, although not fully complete given the ongoing nature of the Covid pandemic, should provide useful information regarding the true extent of Covid infection in New Zealand and possible heart health consequences in the medium-term. Furthermore, the study may provide some answers to obvious questions and issues New Zealanders will have to deal with, such as:
* How do I know my vaccination has worked and given me antibodies? (Symptoms, or the lack of them, after vaccination are not a reliable indicator.)
* How long do measurable antibodies in my blood last? (This question becomes more important given the known breakthrough Covid infections in vaccinated individuals overseas.)
* If I am required to have a booster or variant specific vaccination, when is the best time to do that? Tracking antibody levels and their waning may help with that.
All of these questions are going to require some form of antibody testing to answer them and provide some level of objective certainty to members of the public and the organisations that require such proof of exposure to virus or vaccine. The CANTATA study seeks to address some of these issues and try to help keep New Zealand at the forefront in dealing with the virus.