Adults have on average two colds per year and young children six per year. Therefore, potential interventions to shorten and alleviate common cold symptoms are of great public health importance. Previously, randomized trials have shown that zinc lozenges can shorten the common cold but the composition of the lozenges is crucial for effectiveness [14,23-26]. There is also strong evidence that regular intake of high-doses of vitamin C shortens colds that occur during the supplementation period [27-29], but so far there is no unambiguous evidence that therapeutic vitamin C is effective if started after the onset of symptoms [27]. It seems that the effects of zinc lozenges and vitamin C for the common cold have been ignored not on the basis of evidence from randomized trials, but because of prejudices [28-30].
Carrageenan is a more recent potential treatment for the common cold. Laboratory studies indicate that it has an effect on various respiratory virus infections, including those caused by rhinovirus and influenza A [5-7], but possibly not adenovirus [1,2]. Although laboratory evidence indicating that carrageenan can prevent virus infections traces back to the 1980s, clinical trials have been carried out only since 2010 [9-12].
A previous IPD meta-analysis of two carrageenan trials [10,12] calculated that colds were on average 1.9 days shorter in patients administered nasal carrageenan [13]. However, the meta-analysis did not take account of the fact that there were censored data for 27 patients which means that they did not recover by the end of the follow-up (Fig. 1A). Therefore, the means are undefined and the calculation of the difference in means is inappropriate. In addition, the meta-analysis [13] did not consider the possibility that the effect of carrageenan might be heterogeneous.
In our IPD meta-analysis of the same two trials [10,12] we used Cox regression and quantile regression, both of which take into account the censored observations. We found that there is strong evidence of a treatment effect for nasal carrageenan when colds last over about a week (Fig 1A and 2; Table 2). However, our analysis did not demonstrate that nasal carrageenan had an effect on shorter colds. The heterogeneity in treatment effect indicates that the previously estimated 1.9-day reduction in common cold duration [13] poorly captures the effects of nasal carrageenan (Fig. 2).
In our analysis, we calculated that nasal carrageenan increased the recovery rate by about 50% for all virus-positive participants over the total follow-up period (Table 1). This effect is not as large as the 3-fold increase in recovery rate in five trials with zinc lozenges [24], but important nonetheless.
In quantile regression, we found that carrageenan shortened 13.7-day colds by 3.9 days corresponding to a 28% reduction (Table 2). These estimates of effect apply to 20% of all included participants. In another 20% of participants, 8.8-day colds were reduced by 1.3 days corresponding to a 15% reduction. These can be compared with the 33% average decrease in common cold duration in seven trials with zinc lozenges [25], and the roughly 20% decrease in common cold duration with high vitamin C doses [31]. The benefit of nasal carrageenan was seen only on colds lasting over a week or so. However, assuming a relative effect, a 30% shortening of long two-week colds is a much more important finding than a similar effect for short colds.
The effect of carrageenan on long colds was also analyzed as the risk of the common cold lasting for over 20 days. Nasal carrageenan reduced the risk of such long colds by 71% (Table 3). On the basis of this outcome, one in every 9 patients benefited from carrageenan.
The apparent benefit against long colds is relevant when considering two further trials on nasal iota-carrageenan. The first trial by Eccles administered carrageenan just for 4 days [9]. The second trial by Eccles also administered carrageenan for 4 days, yet patients were allowed to use it for longer; however, there are no data about how long the patients actually used carrageenan in that trial [11]. Furthermore, the two trials followed the patients for just 7 and 10 days, respectively, while the current analysis over 21 days indicates that the greatest benefits may appear only after 7 days (Figs. 1A and 2). Nevertheless, even though ideally the intervention and follow-up periods should have been longer, the two short-term trials also found that carrageenan was beneficial. In the first Eccles trial, the total symptom score over days 2 to 4 was decreased by 26% (P = 0.046) [9], and in the second, the total symptom score over days 1 to 4 was decreased by 9% (P = 0.042) [11]. Reduction in the respiratory virus load has also been observed in the carrageenan participants [9-13].
We also found that carrageenan halved the recurrence of colds during the follow-up period (Table 4). On the basis of this finding, one in every 6 patients benefited from carrageenan. While it is not evident whether the recurrence of symptoms is caused by the same virus or by a new virus, halving the occurrence of new cold-type symptoms in such a large proportion of participants is a clinically relevant finding. Most recurrences occurred after cessation of treatment [13] and therefore administration for longer than 7-days should be tested in further trials to ascertain whether recurrence may be further reduced. Previously, a meta-analysis of four trials in British males found that vitamin C on average halved the recurrence of colds during the follow-up, but that finding may be explained by particularly low dietary vitamin C intakes and is unlikely to be generalizable to the wider population [32]. A more recent trial in the UK also found a significant decrease in the recurrence of colds [33]. To our knowledge there are no data on the possible effects of zinc lozenges on the recurrence of colds.
The common cold is not a homogeneous entity. The majority of common cold symptoms are caused by several different virus types, but the distribution of viruses varies over time and location. In addition, some of the cold-type symptoms are caused by non-viral causes such as allergy. Nevertheless, as regards the currently circulating new coronavirus (SARS-CoV-2), the pattern of findings from carrageenan is particularly interesting. Our analysis gives strong direct evidence that carrageenan is effective against two old coronaviruses OC43 or 229E by increasing the recovery rate by 2.4 fold and by decreasing the recurrence of cold symptoms by 60%. Even the old coronaviruses have caused severe acute respiratory infections [34]. Furthermore, the efficacy against rhinovirus and influenza A virus indicates that the effects are nonspecific (Tables 1 and 4). This does not necessarily mean that carrageenan is effective against the new coronavirus; however, the non-specificity of carrageenan makes it highly plausible that carrageenan will also have an effect on infections caused by the new coronavirus.
The primary outcome in our analysis was self-reported recovery from the common cold in the two included trials [10,12]. Although some researchers may consider that a subjective outcome such as this one is suboptimal, it is the patient who decides whether to visit a physician to ask for a certificate for sick leave or to take time off work because of illness. Diagnosis of the common cold by virology is not feasible because of the large number of viruses and the variability in the disease states caused by the viruses. Given that patients are familiar with the common cold it seems a more reasonable approach to use self-diagnosis for clinical research [35]. Furthermore, the FDA encourages patient-reported outcomes, because they are not biased by the interpretations of physicians or others [36].
Carrageenan has been used as a food component for decades and is classified by the FDA as ‘generally regarded as safe’ [GRAS] [37]. Many concerns about potential harms of carrageenan have been shown to be unfounded and are explained, for example, by confusing carrageenan with polygeenan and using inappropriate biological model systems [37-39]. A recent review concluded that animal studies have found dietary carrageenan to be safe in that it is not a carcinogen or tumor promoter, nor does it have developmental, reproductive or genotoxic effects [37]. The European Food Safety Authority states that ”no adverse effects have been detected in chronic toxicity studies with carrageenan in rats up to 7,500 mg/kg bw per day” [40]. For a 70 kg person, this corresponds to 525 grams per day. Furthermore, the topic of our study is nasally administered carrageenan with a dose of 0.001 g/day for one to two weeks. Therefore potential concerns about high oral doses for decades are not pertinent to the intervention we analyze in this study. A recent study with mice and rabbits indicated that nasal and pulmonary administration of iota-carrageenan does not cause acute adverse effects [41].
All four trials on nasal iota-carrageenan used 3 to 4 daily doses [9-12]. In further research, the dose response should be investigated by testing higher frequency of use to identify the level that leads to maximal effects. Similarly, the dose-response for the amount of carrageenan within the single nasal dose should be examined. Since most recurrences of cold symptoms occurred after cessation of treatment, longer administration should also be tested. It is also possible that the length of time between the onset of symptoms and the start of treatment has an impact on the effectiveness. The two trials included in our meta-analysis specified that treatment should be started within 36-48 hours of symptom onset [10,13], whereas the corresponding time limit in zinc lozenge trials has often been 24 hours [26]. Evidently, the effect of the time between the onset of symptoms and the start of carrageenan treatment on the size of the benefit should be investigated in further trials.
Laboratory studies have found that the combination of carrageenan with oseltamivir and zanamivir has synergistic effects against murine influenza [6,7]. Similarly, it would seem reasonable to examine in a factorial setting the effects of combining carrageenan with zinc lozenges and/or vitamin C since they all have different modes of effect.
In conclusion, we estimate that nasal iota-carrageenan may increase the recovery rate from the common cold by about 50% and shorten the duration of long colds by about 30%. We did not find a beneficial effect on short colds. If able to be replicated, the findings of this study are important for future treatment options for coronavirus and influenza virus infections. Further research should be carried out to examine the effects of nasal iota-carrageenan on respiratory virus infections in more detail.